Differences in the stability of developmentally regulated mRNAs in aggregated and disaggregated Dictyostelium discoideum cells

mRNA stabilization controls the expression of a class of developmentally regulated genes in Dictyostelium discoideum

During the development of Dictyostelium discoideum, several thousand new mRNA species appear in the cytoplasm after the cells have formed stable aggregates. Here we show that six of these late mRNAs, corresponding to six clones randomly chosen from a genomic library, are synthesized from the very beginning of development at a rate comparable to that observed late in development but that transcripts do not accumulate until after aggregation. The early- and late-synthesized mRNAs are identical in size and compete with each other for hybridization to the genomic clones. The early-synthesized mRNAs do not accumulate in the cytoplasm in the preaggregation stage because they are very unstable. Their stability, estimated from the kinetics of incorporation during continuous labeling with (32)P, increases by perhaps an order of magnitude in the postaggregation stage. We conclude that mRNA stabilization is the major controlling factor of the expression of these genes.

Specific cell-cell contact serves as the developmental signal to deactivate discoidin I gene expression in Dictyostelium discoideum

Specific cell-cell contact is a major regulatory signal controlling cell differentiation in Dictyostelium discoideum, causing dramatic changes in the developmental program of gene expression. In this report, we focus on the relationships between specific cell-cell contact and the activity of the genes for discoidin I, an endogenous lectin that has been implicated in the cell-cell cohesion process. By performing quantitative RNA dot-hybridization assays and RNA gel blot-hybridization analyses, using as a probe a recombinant plasmid containing a discoidin I cDNA insert, we have measured changes in discoiding I mRNA levels during normal development and in response to specific manipulations of the state of cellular aggregation. Our major findings are as follows. (i) During normal development on filters, there is a close temporal correspondence between the establishment of specific cell-cell contacts and the decline in discoidin I mRNA levels. By the tight-aggregate stage, discoidin I mRNA is barely detectable. (ii) When tight aggregates are disaggregated and the cells are maintained in the disaggregated state, there is a dramatic rise in discoidin I mRNA content. (iii) When cells are developed in suspension (conditions that interfere with the establishment of tight cell-cell contacts), discoidin I mRNA accumulates to abnormally high levels, and these persist well after the levels in filter-developed cells have declined. Taken together, these results strongly suggest that cell-cell contact is the normal developmental signal to deactivate discoidin I gene expression; thus, a contact-deactivated gene for which a recombinant DNA probe is available has now been identified. Furthermore, we demonstrate that exogenous cAMP almost completely blocks the disaggregation-induced reactivation of discoidin I gene expression. Possible mechanistic relationships between specific cell-cell contact, intracellular cAMP levels, and developmental gene expression are discussed.

Two Dictyostelium ribosomal proteins act as RNases for specific classes of mRNAs

Phosphorylation of ribosomal protein S6 leads to the stabilization of pre-spore specific mRNAs during development of Dictyostelium discoideum. The purification of S6 kinase has allowed the identification of protein S11 as the mRNase specific for pre-spore mRNAs. Methylation of ribosomal protein S31 leads to the destabilization of ribosomal protein mRNAs. The purification of S31 methyltransferase has allowed the identification of protein S29 as the mRNAse specific for ribosomal protein mRNAs.

Synthesis of ribosomal proteins in developing Dictyostelium discoideum cells is controlled by the methylation of proteins S24 and S31

Ribosomal protein mRNAs left over from growth are selectively excluded from polyribosomes in the first half of Dictyostelium discoideum development. This is due to the fact that they are sequestered by a class of free 40S ribosomal subunits, characterized by possessing a methylated S24 protein. At the time of formation of tight cell aggregates, the methylated S24 is substituted by an unmethylated S24, while protein S31 of the same or other 40S subunits becomes methylated. This leads to a rapid degradation of the ribosomal protein mRNAs.

Induction of ribosomal subunits misassembly by antisense RNAs to control cell growth

The assembly of ribosomal subunits starting from free ribosomal RNA and protein of Dictyostelium discoideum was induced in vitro in the presence of several oligoribonucleotides complementary to defined sequences of ribosomal RNA. The reconstituted particles had a full complement of ribosomal proteins, but did not function in an in vitro protein synthesis system and were disassembled following interaction with mRNA. The same result was obtained in vivo by fusing the oligodeossiribonucleotides coding for the selected oligoribonucleotides to the promoter of the gene coding for contact site A protein. This gene is expressed only in the first part of development. Transfected growing cells, transferred in developing buffer in the presence of pulses of cAMP, accumulated significant amounts of the oligoribonucleotides. When retransferred to the growth medium, they grew progressively more slowly, until their doubling time doubled, apparently due to the availability of a limiting amount of functional ribosomes. To avoid disassembly of misassembled subunits (G. Mangiarotti et al., 1997, J. Biol. Chem. 272, 27818-27822), two oligoribonucleotides complementary to sequences present at the 5' ends of pre-17S and pre-26S RNAs were also induced to accumulate during early development with the same technique. When transfected cells were retransferred to the growth medium, their rate of growth declined rapidly to zero and cells died, apparently because they were unable to disassemble misassembled ribosomal subunits and avoid their entry into polyribosomes. This technique to perturb protein synthesis, arrest cell growth, and cause cell suicide will be tested in abnormally growing animal cells.

Cell type specificity and mechanism of control of a gene may be reverted in different strains of Dictyostelium discoideum

Twelve genes which are expressed exclusively in pre-spore cells of Dictyostelium strain AX3 are expressed exclusively in pre-stalk cells of strain AX2. One gene has the opposite behavior: it is expressed in pre-stalk cells in AX3 and in pre-spore cells in AX2. The change in cell type specificity involves a change in the mechanism of control of gene expression. When they are expressed in pre-stalk cells, genes are controlled at the level of transcription, whilst in pre-spore cells, they are controlled at the level of mRNA stability. Genes expressed in pre-stalk cells in strain AX2, fused with an AX2 pre-spore specific promoter, become regulated at the level of mRNA stability. These findings indicate that at least a group of pre-stalk mRNAs possess the cis-destabilizing element typical of pre-spore mRNAs, though they are not destabilized in disaggregated cells. This is due to the fact that ribosomal protein S6, phosphorylation of which is responsible for controlling the stability of pre-spore mRNAs, is not dephosphorylated in disaggregated pre-stalk cells. These cells lack an S6 phosphatase activity which has been purified from disaggregated pre-spore cells.

Ribosomal protein gene expression is cell type specific during development in Dictyostelium discoideum

Starvation for amino acids initiates the developmental cycle in the cellular slime mold, Dictyostelium discoideum. Upon starvation one of the earliest developmental events is the selective loss of the ribosomal protein mRNAs from polysomes. This loss depends upon sequences in the 5' non-translated leader of the ribosomal protein (r-protein) mRNAs. Here evidence is presented which indicates that those cells which will become prestalk cells express the ribosomal protein genes during development under starvation conditions. Cells which enter the prespore pathway shut off r-protein synthesis. The promoter and 5' non-translated leader sequences from two ribosomal protein genes, the rp-L11 and the rp-S9 genes, are fused to the Escherichia coli beta-galactosidase reporter gene. While beta-galactosidase enzyme activity is detected in situ in most growing cells, by 15 h of development beta-galactosidase enzyme activity is largely lost from the prespore cells although strong beta-galactosidase enzyme activity is present in the prestalk cells. These observations suggest the possibility that the ribosomal protein mRNAs are excluded from polysomes in a cell-type-specific manner.

Coupling of transcription and translation in Dictyostelium discoideum nuclei

The nuclei of Dictyostelium discoideum cells have been found to contain polyribosomes active in protein synthesis. mRNA molecules enter nuclear polyribosomes while they are still being synthesized. "Non sense mediated mRNA decay" occurs in the nucleus, through the interaction of the mRNAs containing a nonsense codon with newly formed nuclear ribosomes, rather than with cytoplasmic ribosomes, as previously generally supposed.

The phosphorylation of protein S6 modulates the interaction of the 40 S ribosomal subunit with the 5'-untranslated region of a dictyostelium pre-spore-specific mRNA and controls its stability

AC914 mRNA, a pre-spore-specific mRNA that accumulates only in the post-aggregation stage of development, is transcribed constitutively as shown by nuclear run-off experiments and by fusing its promoter to the luciferase reporter gene. The same mRNA disappears quickly from disaggregated cells. If the 5'-untranslated region (5'UTR) of the constitutively expressed Actin 15 mRNA is substituted for the 5'UTR of AC914 mRNA, this can no longer be destabilized and accumulates both in growing and disaggregated cells. If the 5'UTR of AC914 mRNA is substituted for the 5'UTR of Actin 15 mRNA, the latter accumulates only in aggregated cells. Pactamycin, but not other inhibitors of protein synthesis, prevents AC914 mRNA from being destabilized in disaggregated cells, suggesting a role of 40 S subunits in the destabilization. This has been confirmed by using an in vitro system in which the in vivo stability of different mRNAs is reproduced. A protein kinase A-dependent phosphorylation of ribosomal protein S6 determines whether 40 S subunits are capable or not of destabilizing AC914 mRNA in the in vitro system.

rRNA maturation as a "quality" control step in ribosomal subunit assembly in Dictyostelium discoideum

In Dictyostelium discoideum, newly assembled ribosomal subunits enter polyribosomes while they still contain immature rRNA. rRNA maturation requires the engagement of the subunits in protein synthesis and leads to stabilization of their structure. Maturation of pre-17 S rRNA occurs only after the newly formed 40 S ribosomal particle has entered an 80 S ribosome and participated at least in the formation of one peptide bond or in one translocation event; maturation of pre-26 S rRNA requires the presence on the 80 S particle of a peptidyl-tRNA containing at least 6 amino acids. Newly assembled particles that cannot fulfill these requirements for structural reasons are disassembled into free immature rRNA and ribosomal proteins.

Reconstitution of functional eukaryotic ribosomes from Dictyostelium discoideum ribosomal proteins and RNA

40 and 60 S ribosomal subunits have been reconstituted in vitro from purified ribosomal RNA and ribosomal proteins of Dictyostelium discoideum. The functionality of the reconstituted ribosomes was demonstrated in in vitro mRNA-directed protein synthesis. The reassembly proceeded well with immature precursors of ribosomal RNA but poorly if at all with mature cytoplasmic RNA species. Reassembly also required a preparation of small nuclear RNA(s), acting as morphopoietic factor(s).

Analysis of a novel cyclic Amp inducible prespore gene in Dictyostelium discoideum: evidence for different patterns of cAMP regulation

The D7 cDNA clone hybridizes to a 2.8 kb mRNA which first appears at the mound stage of development in the cellular slime mold Dictyostelium discoideum. This gene which is cyclic AMP (cAMP) inducible and is expressed specifically in the prespore cells contains an open reading frame interrupted by only one intron. The predicted amino acid sequence indicates a novel prespore protein which differs from all of the previously described prespore proteins in that it contains no internal repeats and does not share any homology with any of the other prespore genes. The amino acid sequence predicts a protein of 850 amino acids with a molecular weight of 95,343 daltons and an isoelectric point of 4.25. The protein is very rich in glutamine (13.8%), asparagine (10.6%) and glutamic acid (10.4%) with one potential glycosylation site and 28 possible sites for phosphorylation. The amino terminus is hydrophobic with characteristics of a signal sequence while the entire carboxyl half of the protein is notable for its hydrophilicity. Comparison of cAMP regulation of the D7 gene with the regulation of two other cAMP regulated prespore genes, the PL3(SP87) gene and the Psa(D19), reveals some striking differences. Disaggregation in the presence of cAMP results in transient degradation of mRNA for all three genes. The transcription rate for the D7 and PsA(D19) genes remains relatively unaffected by disaggregation but there is a rapid although transient decline in the transcription rate of the PL3(SP87) gene. Although the accumulation of all three mRNAs is first detectable at mound stage, transcription of the D7 and PsA(D19) genes is detected earlier in development, at rippling aggregate stage several hours prior to the earliest time when transcription of the PL3(SP87) gene is detected. Analysis of the promoter region of the D7 gene reveals three CA like boxes flanked by direct repeats as well as four G rich regions that may serve as regulatory elements.

Involvement of cell-cell adhesion in the expression of the cell cohesion molecule gp80 in Dictyostelium discoideum

Soon after the initiation of the developmental cycle of Dictyostelium discoideum, cells acquire EDTA-sensitive cell-cell binding sites mediated by the glycoprotein gp24. Cells at the aggregation stage display a second type of cell adhesion site, the EDTA-resistant cell-cell binding sites, mediated by the glycoprotein gp80. The gene encoding gp80 is first turned on to a low basal level of expression in the preaggregation stage. At the onset of the aggregation stage, cells produce pulses of low levels of cAMP, which greatly augment the expression of gp80. To investigate the role of cell-cell adhesion in the regulation of gp80 expression, cells were developed in the presence of EDTA or carnitine to block the EDTA-sensitive cell binding sites. Alternatively, cell cohesion was disrupted by shaking low-density cultures at high shearing forces. In all three instances, gp80 was expressed at a substantially reduced level. In addition, exogenous cAMP pulses, which normally were capable of stimulating a precocious and enhanced expression of gp80, failed to restore the high level of gp80 expression. However, if the formation of cell-cell contact was permitted, exogenous cAMP pulses were able to rescue the expression of gp80 even when the cAMP signal relay was blocked. These results indicate that previous cell-cell contact, provided by the EDTA-sensitive binding sites, is required for the activation of the cAMP-mediated signal transduction pathway producing high levels of gp80 expression.

Differential antisense transcription from the Dictyostelium EB4 gene locus: implications on antisense-mediated regulation of mRNA stability

The 2.2 kb mRNA of the Dictyostelium discoideum prespore gene EB4-PSV is constitutively transcribed during growth and development, but the message is only accumulated when cells form aggregates and establish the prespore-prestalk pattern. Disruption of the pattern by mechanical disaggregation results in a rapid loss of the mRNA, while transcription remains nearly unchanged. In early development and after disaggregation, when the mRNA is unstable, a 1.8 kb antisense transcript originating from the same gene locus is detected. This RNA has apparently no coding capacity and is transcriptionally regulated by a promoter located within the translated region of the gene. Excess transcription of antisense RNA in vegetative cells and after disaggregation suggests its involvement in the control of mRNA stability. In agreement with this assumption, the inhibition of RNA synthesis during disaggregation prevents destabilization of the mRNA. This stability regulation of an endogenous mRNA is reminiscent of the loss of specific mRNAs in cells transformed with antisense constructs.

Protein binding and DNase-I-hypersensitive sites in the cis-acting regulatory region of the spore-coat SP96 gene of Dictyostelium

The spore-coat protein gene (SP96) of Dictyostelium discoideum is transcribed only in prespore cells. To identify the cis-acting region of this gene, mutant mini-genes which contained different lengths of 5' upstream region, the partially deleted SP96 coding region and ca. 600 bp of 3' flanking sequence were transformed into D. discoideum cells. Expression of the mini-genes was analysed by Northern hybridization. Our results indicate that the 5' upstream region from -686 to -494 contains an important cis-acting element for the temporal and cell type-specific transcription. A nuclear factor which specifically bound the cis-acting region was identified by gel retardation assay. DNase-I-hypersensitivity of the 5' upstream region was examined and it was shown that the appearance of two new hypersensitive sites correlates with transcriptional activation of the gene. One of the two sites maps to the TATA region and the other was located in the cis-acting region identified by deletion analysis. Our results suggest that gene activation occurs by conformational changes in the chromatin structure of the cis-acting region followed by subsequent binding of regulatory factors and the TATA-binding protein.

Developmental regulation of the alpha-mannosidase gene in Dictyostelium discoideum: control is at the level of transcription and is affected by cell density

In Dictyostelium discoideum, there is a group of genes that are expressed following starvation and when exponentially growing cells reach high densities. We have examined the expression of one of these genes, alpha-mannosidase. Using an alpha-mannosidase cDNA probe in Northern (RNA) blot analysis, we have shown that the previously observed increase in alpha-mannosidase enzyme-specific activity during development is due to an increase in the levels of alpha-mannosidase mRNA. mRNA levels reach a maximum by 8 h of development and then begin to decline by 14 to 22 h. Using nuclear run-on analysis, we have found that this gene is regulated at the level of transcription. We also examined the effects of cell-cell contacts, cyclic AMP levels, and protein synthesis on expression of this gene and found that they were not critical in regulating its expression. However, cell density did play a major role in the expression of alpha-mannosidase. High cell density or the presence of buffer conditioned by high-density cells was sufficient to induce expression of alpha-mannosidase, indicating that this is one of the prestarvation response genes. Finally, the alpha-mannosidase gene was not expressed in aggregation-negative mutant strain HMW 404.

Developmental regulation of the alpha-mannosidase gene in Dictyostelium discoideum: control is at the level of transcription and is affected by cell density

In Dictyostelium discoideum, there is a group of genes that are expressed following starvation and when exponentially growing cells reach high densities. We have examined the expression of one of these genes, alpha-mannosidase. Using an alpha-mannosidase cDNA probe in Northern (RNA) blot analysis, we have shown that the previously observed increase in alpha-mannosidase enzyme-specific activity during development is due to an increase in the levels of alpha-mannosidase mRNA. mRNA levels reach a maximum by 8 h of development and then begin to decline by 14 to 22 h. Using nuclear run-on analysis, we have found that this gene is regulated at the level of transcription. We also examined the effects of cell-cell contacts, cyclic AMP levels, and protein synthesis on expression of this gene and found that they were not critical in regulating its expression. However, cell density did play a major role in the expression of alpha-mannosidase. High cell density or the presence of buffer conditioned by high-density cells was sufficient to induce expression of alpha-mannosidase, indicating that this is one of the prestarvation response genes. Finally, the alpha-mannosidase gene was not expressed in aggregation-negative mutant strain HMW 404.

Dictyostelium discoideum lipids modulate cell-cell cohesion and cyclic AMP signaling

During Dictyostelium discoideum development, cell-cell communication is mediated through cyclic AMP (cAMP)-induced cAMP synthesis and secretion (cAMP signaling) and cell-cell contact. Cell-cell contact elicits cAMP secretion and modulates the magnitude of a subsequent cAMP signaling response (D. R. Fontana and P. L. Price, Differentiation 41:184-192, 1989), demonstrating that cell-cell contact and cAMP signaling are not independent events. To identify components involved in the contact-mediated modulation of cAMP signaling, amoebal membranes were added to aggregation-competent amoebae in suspension. The membranes from aggregation-competent amoebae inhibited cAMP signaling at all concentrations tested, while the membranes from vegetative amoebae exhibited a concentration-dependent enhancement or inhibition of cAMP signaling. Membrane lipids inhibited cAMP signaling at all concentrations tested. The lipids abolished cAMP signaling by blocking cAMP-induced adenylyl cyclase activation. The membrane lipids also inhibited amoeba-amoeba cohesion at concentrations comparable to those which inhibited cAMP signaling. The phospholipids and neutral lipids decreased cohesion and inhibited the cAMP signaling response. The glycolipid/sulfolipid fraction enhanced cohesion and cAMP signaling. Caffeine, a known inhibitor of cAMP-induced adenylyl cyclase activation, inhibited amoeba-amoeba cohesion. These studies demonstrate that endogenous lipids are capable of modulating amoeba-amoeba cohesion and cAMP-induced activation of the adenylyl cyclase. These results suggest that cohesion may modulate cAMP-induced adenylyl cyclase activation. Because the complete elimination of cohesion is accompanied by the complete elimination of cAMP signaling, these results further suggest that cohesion may be necessary for cAMP-induced adenylyl cyclase activation in D. discoideum.

Unexpected localisation of cells expressing a prespore marker of Dictyostelium discoideum

We show that the anterior, prestalk region of the Dictyostelium slug contains cells which express, or have expressed, a prespore-specific marker. We term these cells "prespore-like cells" (PLC). In newly formed slugs there is a sharp prespore/prestalk boundary, with very few PLC, but after several days of migration the clear demarcation between prespore and prestalk zones breaks down because the number of PLC increases dramatically. This is consistent with previous observations showing there to be rapid interchange of cells between the prestalk and prespore regions. This is not, however, their only source, as a scattering of PLC appear when separate prestalk and prespore regions first become apparent at the time of tip formation. Also, at culmination, there is respecification of "prespore" cells at the prestalk/prespore boundary to form part of the mature stalk. The existence of these cells, and of PLC, may explain why we find prespore-specific mRNAs in mature stalk cells.

Dictyostelium discoideum gene family contains a long internal amino acid repeat

Two different cDNA clones denoted pTO270-6 and pTO270-11 represent two mRNAs that are developmentally regulated during spore germination in Dictyostelium discoideum. The respective mRNAs are found only during early germination and are not present in other stages of growth or multicellular development. Four different genomic clones that hybridize to sequences that are common to both of the 270 cDNA clones were isolated from Dictyostelium libraries and sequenced. Two are the genes for the two cDNAs, and the other two represent genes that do not seem to be transcribed. All four genomic sequences possess a very unusual internal feature in the deduced protein sequences composed of a monotonous repeat of the tetrapeptide threonine-glutamic acid-threonine-proline. The other portions of the proteins have no homology among themselves. The deduced protein corresponding to the 270-6 gene is very similar to avocado (Persea americana) cellulase. Since cellulose in the spore wall has to be digested during spore germination this suggests that this protein may function as an endo-(1,4)-beta-D-glucanase during germination.

Developmental protein synthesis is required for the transcription of Dictyostelium prespore genes

It has been established previously that the maintenance of expression of prespore-specific genes of Dictyostelium discoideum is prevented by the translational inhibitor cycloheximide. The drug had no effect upon the level of transcripts of the other genes examined, prestalk-specific or cell type-nonspecific. However, the interpretation of this result is open to question, because of possible nonspecific effects of cycloheximide. We have now characterized the cellular specificity and temporal profiles of mRNA accumulation of additional Dictyostelium cDNA clones, and have examined other inhibitors of in vivo protein synthesis. Four structurally and mechanistically distinct translational inhibitors each prevented the reaccumulation of prespore transcripts in cyclic AMP-primed, disaggregated amoebae. These results establish the importance of developmental protein synthesis in the accumulation of prespore gene transcripts. Nuclear run-on transcription assays were used to learn whether protein synthesis is required primarily for mRNA synthesis or transcript stability. A transcriptional time course first demonstrated that the abundance of these cell-specific transcripts during development mirrors their rates of synthesis. Significantly, the protein synthesis requirement of the prespore genes examined also occurs at the level of mRNA transcription, implying the existence of one or more developmentally regulated transcriptional activators.

Dictyostelium discoideum lipids modulate cell-cell cohesion and cyclic AMP signaling

During Dictyostelium discoideum development, cell-cell communication is mediated through cyclic AMP (cAMP)-induced cAMP synthesis and secretion (cAMP signaling) and cell-cell contact. Cell-cell contact elicits cAMP secretion and modulates the magnitude of a subsequent cAMP signaling response (D. R. Fontana and P. L. Price, Differentiation 41:184-192, 1989), demonstrating that cell-cell contact and cAMP signaling are not independent events. To identify components involved in the contact-mediated modulation of cAMP signaling, amoebal membranes were added to aggregation-competent amoebae in suspension. The membranes from aggregation-competent amoebae inhibited cAMP signaling at all concentrations tested, while the membranes from vegetative amoebae exhibited a concentration-dependent enhancement or inhibition of cAMP signaling. Membrane lipids inhibited cAMP signaling at all concentrations tested. The lipids abolished cAMP signaling by blocking cAMP-induced adenylyl cyclase activation. The membrane lipids also inhibited amoeba-amoeba cohesion at concentrations comparable to those which inhibited cAMP signaling. The phospholipids and neutral lipids decreased cohesion and inhibited the cAMP signaling response. The glycolipid/sulfolipid fraction enhanced cohesion and cAMP signaling. Caffeine, a known inhibitor of cAMP-induced adenylyl cyclase activation, inhibited amoeba-amoeba cohesion. These studies demonstrate that endogenous lipids are capable of modulating amoeba-amoeba cohesion and cAMP-induced activation of the adenylyl cyclase. These results suggest that cohesion may modulate cAMP-induced adenylyl cyclase activation. Because the complete elimination of cohesion is accompanied by the complete elimination of cAMP signaling, these results further suggest that cohesion may be necessary for cAMP-induced adenylyl cyclase activation in D. discoideum.

Cell-type-specific genes expressed late in Dictyostelium development show markedly different responses to 3'5' cyclic AMP

The maintenance of late gene expression by 3'5' cyclic AMP was re-examined using several newly isolated cell-type-specific genes. Expression of all the prespore-enriched genes ceased immediately upon disaggregation of developing cells and pre-existing mRNA was rapidly degraded. For most genes, cAMP had little or no effect either alone or in combination with conditioned medium factors. The expression of the non-cell-type-specific genes 7E and 2C also ceased upon cell disaggregation but cAMP triggered a full re-induction of expression although the timing of the response differed markedly between these two genes. In contrast to earlier interpretations, these data argue that for none of these late prespore genes is cAMP alone sufficient for the maintenance of expression. The responses of the two prestalk mRNAs examined were gene-specific. Prestalk 5D mRNA decayed slowly upon disaggregation and was partially stabilized by cAMP whereas prestalk 5G mRNA increased upon disaggregation and was inhibited by cAMP.

Developmental regulation of cell-type-enriched mRNAs in Dictyostelium discoideum

We describe sixteen new families of cDNA clones representing mRNAs that are expressed preferentially in either prespore or prestalk cells during development of Dictyostelium discoideum and two new mRNAs that are expressed in a non-cell-type-specific manner. None of the prespore-enriched mRNAs are detectable in Dictyostelium cells until 13-15 h of development but then they increase dramatically and peak at 18-22 h. Upon dissociation of developing aggregates, all these mRNAs rapidly decay to low levels. In marked contrast to data presented for prespore genes by other workers, cyclic AMP either has no effect on the mRNA levels in dissociated cells or is only weakly effective in restoring normal expression. A prestalk-enriched mRNA examined, 5G mRNA, is similarly expressed late in development but is also expressed in vegetative cells. The level of 5G mRNA is only moderately affected by cell disaggregation.

Analysis of developmentally expressed antigens in Polysphondylium pallidum

A series of monoclonal antibodies were previously raised against developing Polysphondylium pallidum cells. In this work, six of these antibodies have been used as probes to identify and characterize antigens regulated during development. Soluble and membrane fractions of P. pallidum cells at six stages of development or three stages of cyclic adenosine monophosphate (cAMP)-induced development were run in sodium dodecyl sulfate (SDS)-polyacrylamide gels and subjected to Western blot analysis. Three of the monoclonals, anti-Tp200, anti-Tp423, and anti-Pg101, stain sorogen tips. Tp423 and Tp200 are membrane-associated antigens; both are stable to urea extraction, and Tp200 remains in the membrane after NaOH extraction. Tp423 is present in starved cells but is more prominent in sorogens and particularly in cAMP-developed cells. In contrast, Tp200 is first detected in early to mid-aggregation and is more abundant late in development. Pg101, which is expressed as a gradient with its highest concentration in tips, first appears in tight aggregates but is much more abundant in sorogens; unlike the Tp antigens, Pg101 is not greatly induced in cAMP-developed cells. All three of these antigens undergo changes in apparent molecular weight at the tight aggregate or sorogen stage: The gel mobilities of Tp200 and Pg101 increase, whereas that of Tp423 decreases. In addition to the tip-specific monoclonals, two monoclonals that stain all but the tips of sorogens have been used for analysis. One of these, anti-3D10Pnk stains most cells within secondary tips, whereas anti-3D10Dif does not. 3D10Dif is membrane associated; it is present very early in development, increasing two- to threefold through the sorogen stage and diminishing in late cAMP-developed cells. 3D10Pnk is a mostly soluble species first detected in late streaming. Anti-1c3, a sixth monoclonal, which stains nuclei uniformly throughout sorogens, is also developmentally expressed. 1c3 is mainly membrane associated and is expressed from late streaming through the sorogen stage.

Two cyclic AMP-regulated genes from Dictyostelium discoideum encode homologous proteins

Expression of the 7E and 2C genes late in Dictyostelium development ceases upon cell disaggregation but, in contrast to many other genes we have studied, expression is fully restored by exogenous cAMP (A. J. Richards et al., submitted). The 7E and 2C genes encode polypeptides of similar size (9220 and 10573 Daltons, respectively), each of which contains an unusually high proportion of serine plus glycine residues (41% and 59%, respectively). Each protein possesses a relatively serine-rich N-terminus and glycine-rich C-terminus and contains the conserved sequence S(X)SSS(X2)SS(X)SS(X2)SFGS. These data suggest that genes 7E and 2C may have arisen by duplication of a common ancestor. Computer analysis indicates that both gene products are probably intracellular structural proteins that form extended coil structures.

Identification of sequences regulating the transcription of a Dictyostelium gene selectively expressed in prespore cells

There has been considerable debate about the relative contributions of transcriptional and post-transcriptional mechanisms to the regulation of prespore gene expression in Dictyostelium. We have determined the DNA sequence upstream of D19, the Dictyostelium gene encoding PsA, a prespore-specific, cell surface protein of unknown function. Our analysis of gene fusions, in which D19 upstream sequences are placed adjacent to a heterologous reporter gene, indicates that transcriptional signals alone are sufficient for the correct temporal and cell-type specific expression of this gene. We also show that the 5' and 3' boundaries of the minimal sequences necessary for correct developmental regulation lie within the region 338 to 122 nucleotides upstream of the start site of transcription but that flanking sequences seem to be necessary for optimal expression.

Conditions that elevate intracellular cyclic AMP levels promote spore formation in Dictyostelium

We have been using sporogenous mutants of Dictyostelium discoideum strain V12M2 to study regulation of cell fate during terminal differentiation of spores and stalk cells. Analyses of intracellular cAMP accumulation, cAMP secretion, cAMP binding to cell surface receptors, and chemotactic sensitivity to exogenous cAMP during aggregation showed that all of these functions were identical in V12M2 and HB200, a sporogenous mutant. We used several methods of altering intracellular cAMP levels in HB200 cells to test the hypothesis that intracellular cAMP levels affect cell fate. First, HB200 amoebae were treated with 5 mM caffeine for 4 h during growth, washed, and allowed to develop in the absence of caffeine. Treated cells had normal levels of intracellular cAMP and adenylate cyclase activities at the beginning of differentiation; by 6 h development, they contained two to three times more intracellular cAMP and two times more GTP-dependent adenylate cyclase activity than untreated cells. However, their level of basal Mn++-dependent adenylate cyclase activity was the same as untreated controls. Thus, treatment of growing HB200 amoebae with caffeine for only 4 h leads to hyperinduction of a GTP-dependent regulator (or inhibition of a negative regulator) of adenylate cyclase during subsequent differentiation, without induction of basal activity. The fraction of amoebae forming spores increased twofold when HB200 amoebae were treated with caffeine during growth. Spore (but not stalk cell) differentiation by such treated cells was blocked by inhibitors of cAMP accumulation. Second, cells grown on nutrient agar accumulated higher levels of intracellular cAMP and formed more spores in vitro than cells grown in shaken suspension.(ABSTRACT TRUNCATED AT 250 WORDS)

Prespore gene expression in Dictyostelium requires concomitant protein synthesis

It has been established previously that the maintenance of expression of prespore-specific genes of Dictyostelium discoideum is prevented by the translational inhibitor cycloheximide. The drug had no effect upon the level of transcripts of the other genes examined, prestalk-specific or cell type non-specific (Mehdy, M., Ratner, D. and Firtel, R., (1983) Cell 32, 763-771). We have now characterized the cellular specificity and temporal profiles of mRNA accumulation of additional Dictyostelium cDNA clones. Other inhibitors of in vivo protein synthesis have been examined, with emetine shown to be a particularly effective but reversible agent. Four structurally and mechanistically distinct translational inhibitors each prevented the reaccumulation of prespore transcripts in cyclic AMP-primed disaggregated amoebae. These results establish a role for protein synthesis in the transcription or transcript stability of prespore genes.

The effects of transcription on the nucleosome structure of four Dictyostelium genes

Micrococcal nuclease digestion of Dictyostelium discoideum nuclei from various developmental stages was used to investigate transcription-related changes in the chromatin structure of the coding region of four genes. Gene activity was determined by Northern blotting and nuclear run on experiments. During strong transcription of the developmentally regulated cysteine proteinase I gene, a smear superimposed on a nucleosomal ladder was observed, indicating perturbation of nucleosomal structure was occurring. However, two other developmentally regulated genes, discoidin I and pSC253, showed only slight nucleosome disruption during high levels of transcription. The chromatin structure of a fourth gene (pCZ22) was disrupted throughout development, even at those stages where transcription was greatly reduced. We suggest that although nucleosome structure can be transiently perturbed by the passage of the transcription complex in vivo, the degree of perturbation and the speed with which nucleosomes reassemble is also influenced by the DNA sequence.

Organization of a gene family developmentally regulated during Dictyostelium discoideum spore germination

mRNA specific to cDNA clone pLK109 is present in Dictyostelium discoideum spores, increases about two- to threefold at 0.5 to 1 h during spore germination, and then rapidly decreases. The mRNA is not detectable in vegetative cells or in early multicellular development on filters, but is present late during development, approximately at the time of sporulation. 109 mRNA in spores is 700 nucleotides in length but this is processed during germination by shortening of the poly(A) tail to about 600 nucleotides at 1 to 1.5 hours. pLK109 is a member of a multigene family containing three separate genes, and we have isolated and sequenced all of them. All three sequences code for deduced proteins of 127 amino acid residues, with only a few amino acid differences among them. Gene 1 represents the "transcribed" gene, since all 33 cDNAs we isolated are identical with the cDNA pLK109 and the coding region of this gene. Other open reading frames are in close proximity to each of the 109 sequences. About 200 base-pairs 3' to the gene 1 109 sequence is an open reading frame in the opposite orientation. Gene 2 fragment contains a sequence that codes for a protein similar to trypanosome alpha-tubulin 728 base-pairs 5' to the 109 sequence. Gene 3 fragment possesses two additional putative coding regions, one 5' and another 3' to the 109 gene. There is a remarkable similarity between the 5' upstream regions of all three genes. Each possesses a normal Dictyostelium TATA box and the usual T stretch. In addition, there are many other portions of about 400 to 500 base-pairs of the 5' regions that are either identical for long stretches or very similar.

A colony-blot technique for the detection of specific transcripts in eukaryotes

We present a simple and rapid technique for the detection of specific transcripts in eukaryotic cells. The method allows the screening of large numbers of clones for the expression of a gene of interest, similar to the colony blotting techniques described for prokaryotes. We have used this method to monitor developmentally regulated transcription of endogenous genes and the expression of foreign genes in transformed Dictyostelium discoideum cells. The same procedure can be applied to detect specific transcripts in yeast and should thus provide a useful molecular tool for most biological systems.

mRNA decay rates in late-developing Dictyostelium discoideum cells are heterogeneous, and cyclic AMP does not act directly to stabilize cell-type-specific mRNAs

We reevaluated the use of 32PO4 pulse-chases for analyzing mRNA decay rates in late-developing Dictyostelium cells. We found that completely effective PO4 chases could not be obtained in developing cells and that, as a consequence, the decay rates exhibited by some mRNAs were influenced by the rates at which they were transcribed. In developing cells disaggregated in the presence of cyclic AMP, the poly(A)+ mRNA population turned over with an apparent half-life of 4 h, individual mRNA decay rates were heterogeneous, and some prestalk and prespore mRNAs appeared to decay with biphasic kinetics. In cells disaggregated in the absence of cyclic AMP, all prestalk and prespore mRNAs decayed with biphasic kinetics. During the first 1 to 1.5 h after disaggregation in the absence of cyclic AMP, the cell-type-specific mRNAs were selectively degraded, decaying with half-lives of 20 to 30 min; thereafter, the residual prestalk and prespore mRNA molecules decayed at rates that were similar to those measured in the presence of cyclic AMP. This short-term labilization of cell-type-specific mRNAs was observed even for those species not requiring cyclic AMP for their accumulation in developing cells. The observation that cell-type specific mRNAs can decay at similar rates in disaggregated cells with or without cyclic AMP indicates that this compound does not act directly to stabilize prestalk and prespore mRNAs during development and that its primary role in the maintenance of cyclic-AMP-dependent mRNAs is likely to be transcriptional.

mRNA decay rates in late-developing Dictyostelium discoideum cells are heterogeneous, and cyclic AMP does not act directly to stabilize cell-type-specific mRNAs

We reevaluated the use of 32PO4 pulse-chases for analyzing mRNA decay rates in late-developing Dictyostelium cells. We found that completely effective PO4 chases could not be obtained in developing cells and that, as a consequence, the decay rates exhibited by some mRNAs were influenced by the rates at which they were transcribed. In developing cells disaggregated in the presence of cyclic AMP, the poly(A)+ mRNA population turned over with an apparent half-life of 4 h, individual mRNA decay rates were heterogeneous, and some prestalk and prespore mRNAs appeared to decay with biphasic kinetics. In cells disaggregated in the absence of cyclic AMP, all prestalk and prespore mRNAs decayed with biphasic kinetics. During the first 1 to 1.5 h after disaggregation in the absence of cyclic AMP, the cell-type-specific mRNAs were selectively degraded, decaying with half-lives of 20 to 30 min; thereafter, the residual prestalk and prespore mRNA molecules decayed at rates that were similar to those measured in the presence of cyclic AMP. This short-term labilization of cell-type-specific mRNAs was observed even for those species not requiring cyclic AMP for their accumulation in developing cells. The observation that cell-type specific mRNAs can decay at similar rates in disaggregated cells with or without cyclic AMP indicates that this compound does not act directly to stabilize prestalk and prespore mRNAs during development and that its primary role in the maintenance of cyclic-AMP-dependent mRNAs is likely to be transcriptional.

Determinants of mRNA stability in Dictyostelium discoideum amoebae: differences in poly(A) tail length, ribosome loading, and mRNA size cannot account for the heterogeneity of mRNA decay rates

As an approach to understanding the structures and mechanisms which determine mRNA decay rates, we have cloned and begun to characterize cDNAs which encode mRNAs representative of the stability extremes in the poly(A)+ RNA population of Dictyostelium discoideum amoebae. The cDNA clones were identified in a screening procedure which was based on the occurrence of poly(A) shortening during mRNA aging. mRNA half-lives were determined by hybridization of poly(A)+ RNA, isolated from cells labeled in a 32PO4 pulse-chase, to dots of excess cloned DNA. Individual mRNAs decayed with unique first-order decay rates ranging from 0.9 to 9.6 h, indicating that the complex decay kinetics of total poly(A)+ RNA in D. discoideum amoebae reflect the sum of the decay rates of individual mRNAs. Using specific probes derived from these cDNA clones, we have compared the sizes, extents of ribosome loading, and poly(A) tail lengths of stable, moderately stable, and unstable mRNAs. We found (i) no correlation between mRNA size and decay rate; (ii) no significant difference in the number of ribosomes per unit length of stable versus unstable mRNAs, and (iii) a general inverse relationship between mRNA decay rates and poly(A) tail lengths. Collectively, these observations indicate that mRNA decay in D. discoideum amoebae cannot be explained in terms of random nucleolytic events. The possibility that specific 3'-structural determinants can confer mRNA instability is suggested by a comparison of the labeling and turnover kinetics of different actin mRNAs. A correlation was observed between the steady-state percentage of a given mRNA found in polysomes and its degree of instability; i.e., unstable mRNAs were more efficiently recruited into polysomes than stable mRNAs. Since stable mRNAs are, on average, "older" than unstable mRNAs, this correlation may reflect a translational role for mRNA modifications that change in a time-dependent manner. Our previous studies have demonstrated both a time-dependent shortening and a possible translational role for the 3' poly(A) tracts of mRNA. We suggest, therefore, that the observed differences in the translational efficiency of stable and unstable mRNAs may, in part, be attributable to differences in steady-state poly(A) tail lengths.

Determinants of mRNA stability in Dictyostelium discoideum amoebae: differences in poly(A) tail length, ribosome loading, and mRNA size cannot account for the heterogeneity of mRNA decay rates

As an approach to understanding the structures and mechanisms which determine mRNA decay rates, we have cloned and begun to characterize cDNAs which encode mRNAs representative of the stability extremes in the poly(A)+ RNA population of Dictyostelium discoideum amoebae. The cDNA clones were identified in a screening procedure which was based on the occurrence of poly(A) shortening during mRNA aging. mRNA half-lives were determined by hybridization of poly(A)+ RNA, isolated from cells labeled in a 32PO4 pulse-chase, to dots of excess cloned DNA. Individual mRNAs decayed with unique first-order decay rates ranging from 0.9 to 9.6 h, indicating that the complex decay kinetics of total poly(A)+ RNA in D. discoideum amoebae reflect the sum of the decay rates of individual mRNAs. Using specific probes derived from these cDNA clones, we have compared the sizes, extents of ribosome loading, and poly(A) tail lengths of stable, moderately stable, and unstable mRNAs. We found (i) no correlation between mRNA size and decay rate; (ii) no significant difference in the number of ribosomes per unit length of stable versus unstable mRNAs, and (iii) a general inverse relationship between mRNA decay rates and poly(A) tail lengths. Collectively, these observations indicate that mRNA decay in D. discoideum amoebae cannot be explained in terms of random nucleolytic events. The possibility that specific 3'-structural determinants can confer mRNA instability is suggested by a comparison of the labeling and turnover kinetics of different actin mRNAs. A correlation was observed between the steady-state percentage of a given mRNA found in polysomes and its degree of instability; i.e., unstable mRNAs were more efficiently recruited into polysomes than stable mRNAs. Since stable mRNAs are, on average, "older" than unstable mRNAs, this correlation may reflect a translational role for mRNA modifications that change in a time-dependent manner. Our previous studies have demonstrated both a time-dependent shortening and a possible translational role for the 3' poly(A) tracts of mRNA. We suggest, therefore, that the observed differences in the translational efficiency of stable and unstable mRNAs may, in part, be attributable to differences in steady-state poly(A) tail lengths.

Specific mRNA destabilization in Dictyostelium discoideum requires RNA synthesis

We tested the effects of inhibitors of protein and RNA synthesis on the disaggregation-mediated destabilization of prespore mRNAs in Dictyostelium discoideum. Incubating disaggregated cells with daunomycin to inhibit RNA synthesis prevented the loss of prespore mRNAs, whereas the inhibitor decreased or did not affect levels of the common mRNAs CZ22 and actin. Protein synthesis inhibitors varied in their effects. Cycloheximide, which inhibited protein synthesis almost completely, prevented the loss of the prespore mRNAs, but puromycin, which inhibited protein synthesis less well, did not. These results indicate that the process of specific mRNA destabilization requires the synthesis of RNA and possibly of protein.

Structural and functional characterization of genes encoding Dictyostelium prestalk and prespore cell-specific proteins

The nucleotide sequence of D19, a Dictyostelium gene that encodes a prespore-specific mRNA sequence shows it to encode PsA, the cell surface protein detected by the MUD 1 monoclonal antibody. The predicted sequence of the protein reveals a largely hydrophobic C terminus, with chemical similarity to proteins known to be attached to the plasma membrane via a phosphatidylinositol link. The C-terminal region has direct sequence homology to the contact sites A protein and to the phosphatidylinositol-linked form of a chicken N-CAM, suggesting that it might play a role in cell adhesion. Expression of the D19 gene is known to be induced by cAMP and repressed by adenosine. The accumulation of the D19 mRNA is also repressed by DIF, the putative stalk-specific morphogen, and this effect is mediated at the transcriptional level. The pDd56 and pDd63 genes are induced by DIF, and they are specific markers of prestalk and stalk cells. They encode, respectively, ST310 and ST430, two proteins that were first identified by two-dimensional gel electrophoresis. Both proteins are predominantly composed of a highly conserved, 24-amino acid repeat. The two proteins are localized in the slime sheath of the migratory slug and in the stalk tube and stalk cell wall of the mature culminant, where they presumably function as structural components of the extracellular matrix. We have constructed marked derivatives of the pDd56, pDd63, and D19 genes, and these are correctly regulated after transformation into Dictyostelium cells. Thus we have determined the structure, and elucidated possible functions, for one prespore and two prestalk genes. These sequences should be of value, both as markers of the earliest events in cellular differentiation and in identifying the regulatory sequences controlling cell type-specific gene expression.

Regulation of mRNA stability and the poly(A) problem in Dictyostelium discoideum

This paper reviews our studies of three aspects of post-transcriptional regulation in Dictyostelium discoideum: 1) the determinants of mRNA stability in vegetative amoebae; 2) the effects of disaggregation and cyclic AMP on the decay rates of cell-type-specific mRNAs in late developing cells; and 3) the cytoplasmic function of the 3' poly(A) tracts present on most mRNAs. We find that: 1) mRNA stability in vegetative amoebae is not dependent on mRNA size, ribosome loading, or poly(A) tract length, but may be determined by specific 3'-untranslated sequences within a given mRNA; 2) mRNA decay rates in late developing cells are heterogeneous, and cyclic AMP does not act directly to stabilize cell-type-specific mRNAs; and 3) poly(A) is most likely involved in the initiation of protein synthesis via an interaction with cytoplasmic poly(A)-binding proteins.

Contact alters cAMP metabolism in aggregation-competent Dictyostelium amoebae

cAMP and cell-cell contact are involved in the coordination of differentiation and morphogenesis in Dictyostelium discoideum. The experiments described in this paper establish a relationship between cAMP and cell-cell contact. Contact between Enterobacter aerogenes and aggregation-competent Dictyostelium amoebae and contact between Dictyostelium amoebae themselves results in the transient secretion of cAMP and an alteration in the amount of cAMP secreted in response to subsequent stimulation by cAMP, i.e., an alteration in magnitude of a cAMP relay response. The subsequent cAMP relay response can be enhanced or diminished depending upon the number of contacts formed and the concentration of cAMP present at the time of contact. Latex beads are capable of evoking cAMP secretion. However, the bead/amoebal contact is unable to alter the magnitude of a subsequent response to cAMP. This suggests that a nonspecific interaction via cell-cell contact elicits transient cAMP secretion in aggregation-competent Dictyostelium amoebae. The two responses to cell-cell contact are distinct from each other and distinct from the cAMP relay response. 1) The dose-response curves for the responses to Enterobacter contact are clearly different. 2) Contact with latex beads can elicit cAMP secretion but not alter the magnitude of a subsequent cAMP relay response. 3) The temperature dependences of the contact-induced responses and the cAMP relay response show that only the contact-induced cAMP secretion is inhibited at 12 and 15 degrees C, while only the cAMP relay response is inhibited at 28 degrees C. A 4-second application of cAMP at the time that contact is initiated enhances both contact-induced responses. Whether the relationship between these two developmental regulators is important for the regulation of Dictyostelium development has yet to be established.

Specific mRNA destabilization in Dictyostelium discoideum requires RNA synthesis

We tested the effects of inhibitors of protein and RNA synthesis on the disaggregation-mediated destabilization of prespore mRNAs in Dictyostelium discoideum. Incubating disaggregated cells with daunomycin to inhibit RNA synthesis prevented the loss of prespore mRNAs, whereas the inhibitor decreased or did not affect levels of the common mRNAs CZ22 and actin. Protein synthesis inhibitors varied in their effects. Cycloheximide, which inhibited protein synthesis almost completely, prevented the loss of the prespore mRNAs, but puromycin, which inhibited protein synthesis less well, did not. These results indicate that the process of specific mRNA destabilization requires the synthesis of RNA and possibly of protein.

Structural and functional characterization of a Dictyostelium gene encoding a DIF inducible, prestalk-enriched mRNA sequence

The pDd56 mRNA sequence is highly enriched in prestalk over prespore cells and is inducible by DIF, the putative Dictyostelium stalk-specific morphogen. We show that the pDd56 gene is composed of forty one copies of a twenty four amino acid, cysteine rich repeat. This is highly homologus to a repeat which we have previously shown to compose the major fraction of the pDd63 mRNA, another DIF inducible, prestalk-enriched sequence. The predicted pDd56 protein contains a putative signal peptide but does not appear to contain a transmembrane segment. In combination these features suggest it to be an extrinsic protein and we confirm this elsewhere by showing that the pDd56 gene encodes a known, extracellular protein of the stalk. The pDd56 mRNA is dependent upon exogenous DIF for its accumulation. We show that this control is exerted at the transcriptional level and that a restriction fragment containing 1.7Kb of upstream sequence directs temporally-regulated expression of the gene.

RNA turnover in Trypanosoma brucei

Regulation of variant surface glycoprotein (VSG) mRNA turnover in Trypanosoma brucei was studied in bloodstream forms, in procyclic cells, and during in vitro transformation of bloodstream forms to procyclic cells by approach-to-equilibrium labeling and pulse-chase experiments. Upon initiation of transformation at 27 degrees C in the presence of citrate-cis-aconitate, the half-life of VSG mRNA was reduced from 4.5 h in bloodstream forms to 1.2 h in transforming cells. Concomitantly, an approximately 25-fold decrease in the rate of transcription was observed, resulting in a 100-fold reduction in the steady-state level of de novo-synthesized VSG mRNA. This low level of expression was maintained for at least 7 h, finally decreasing to an undetectable level after 24 h. Transcription of the VSG gene in established procyclic cells was undetectable. For comparison, the turnover of polyadenylated and nonpolyadenylated RNA, beta-tubulin mRNA, and mini-exon-derived RNA (medRNA) was studied. For medRNA, no significant changes in the rate of transcription or stability were observed during differentiation. In contrast, while the rate of transcription of beta-tubulin mRNA in in vitro-cultured bloodstream forms, transforming cells, and established procyclic cells was similar, the half life was four to five times longer in procyclic cells (t1/2, 7 h) than in cultured bloodstream forms (t1/2, 1.4 h) or transforming cells (t1/2, 1.7 h). Inhibition of protein synthesis in bloodstream forms at 37 degrees Celsius caused a dramatic 20-fold decrease in the rate of VSG mRNA synthesis and a 6-fold decrease in half-life to 45 min, while beta-tubulin mRNA was stabilized 2- to 3-fold and mRNA stability remained unaffected. It is postulated that triggering transformation or inhibiting protein synthesis induces changes in the abundance of the same regulatory molecules which effect the shutoff of VSG gene transcription in addition to shortening the half-life of VSG mRNA.

Role of protein synthesis in decay and accumulation of mRNA during spore germination in the cellular slime mold Dictyostelium discoideum

Spore germination in Dictyostelium discoideum is a particularly suitable model for studying the regulation of gene expression, since developmentally regulated changes in both protein and mRNA synthesis occur during the transition from dormant spore to amoeba. The previous isolation of three cDNA clones specific for mRNA developmentally regulated during spore germination allowed for the quantitation of the specific mRNAs during this process. The three mRNAs specific to clones pLK109, pLK229, and pRK270 have half-lives much shorter (minutes) than those of constitutive mRNAs (hours). Using spore germination as a model, we studied the roles of ribosome-mRNA interactions and protein synthesis in mRNA degradation by using antibiotics that inhibit specific reactions in protein biosynthesis. Cycloheximide inhibits the elongation step of protein synthesis. Polysomes accumulate in inhibited cells because ribosomes do not terminate normally and new ribosomes enter the polysome, eventually saturating the mRNA. Pactamycin inhibits initiation, and consequently polysomes break down in the presence of this drug. Under this condition, the mRNA is essentially free of ribosomes. pLK109, pLK229, and pRK270 mRNAs were stabilized in the presence of cycloheximide, but pactamycin had no effect on their normal decay. Since it seems likely that stability of mRNA reflects the availability of sites for inactivation by nucleases, it follows that in the presence of cycloheximide, these sites are protected, presumably by occupancy by ribosomes. No ribosomes are bound to mRNA in the presence of pactamycin, and therefore mRNA degrades at about the normal rate. The data further indicate that a labile protein is probably not involved in mRNA decay or stabilization, since protein synthesis is inhibited equally by both antibiotics. We conclude that it may be important to use more than one type of protein synthesis inhibitor to evaluate whether protein synthesis is required for mRNA decay. The effect of protein synthesis inhibition on mRNA synthesis and accumulation was also studied. mRNA synthesis continues in the presence of inhibitors, albeit at a diminished rate relative to that of the uninhibited control.

Gene regulation during dedifferentiation in Dictyostelium discoideum

During development of Dictyostelium discoideum, cells acquire the capacity to rapidly recapitulate morphogenesis. Therefore, when cells at the loose aggregate stage are disaggregated and challenged to reaggregate, they do so in a tenth of the original time. If loose aggregate cells are disaggregated and resuspended in buffered dextrose solution (erasure medium), they retain the capacity of rapid recapitulation for 80 min, then completely lose this capacity in a single, synchronous step referred to as the "erasure event." The erasure event sets in motion a program of dedifferentiation during which cells lose developmentally acquired characteristics at different times. The erasure event is inhibited by the addition of 10(-4) M cAMP to erasure medium. The synthesis of 33 growth-associated polypeptides, the synthesis of 53 development-associated polypeptides, and the level of 2 development-associated RNAs have been monitored during the erasure program and in cultures inhibited from erasing by the addition of 10(-4) M cAMP. Growth-associated polypeptides begin to be resynthesized and development-associated polypeptides exhibit dramatic decreases in rate of synthesis at different times throughout the first 240 min in erasure medium. Inhibiting the erasure event with cAMP has no major effect in the resynthesis of the majority of growth-associated polypeptides. Only one growth-associated polypeptide, V28, is completely inhibited by cAMP, suggesting that it may play a role in the erasure process. In contrast, inhibiting the erasure event with cAMP has a marked effect on the synthesis of development-associated polypeptides, causing a dramatic reduction in the rate at which synthesis decreases for 6 polypeptides, and completely inhibits the decrease in the synthetic rate of 8 polypeptides. The two development-associated RNAs, 16G1 and 10C3, exhibit two distinctly different patterns of loss during erasure, but in both cases cAMP added at time zero of the erasure process dramatically retards or inhibits loss. In addition, when cAMP is added just prior to the erasure event, it inhibits the erasure event and stimulates a rapid increase in the level of 16G1 RNA back to the developmental level. The level of 16G1 RNA then remains at this level for at least 400 min. When cAMP is added after the erasure event, it causes a low, transient increase in the level of 16G1 RNA. These results are considered both in relation to the program of erasure, and in relation to the role of cAMP in the expression of developmental genes during the forward program of development.

RNA turnover in Trypanosoma brucei

Regulation of variant surface glycoprotein (VSG) mRNA turnover in Trypanosoma brucei was studied in bloodstream forms, in procyclic cells, and during in vitro transformation of bloodstream forms to procyclic cells by approach-to-equilibrium labeling and pulse-chase experiments. Upon initiation of transformation at 27 degrees C in the presence of citrate-cis-aconitate, the half-life of VSG mRNA was reduced from 4.5 h in bloodstream forms to 1.2 h in transforming cells. Concomitantly, an approximately 25-fold decrease in the rate of transcription was observed, resulting in a 100-fold reduction in the steady-state level of de novo-synthesized VSG mRNA. This low level of expression was maintained for at least 7 h, finally decreasing to an undetectable level after 24 h. Transcription of the VSG gene in established procyclic cells was undetectable. For comparison, the turnover of polyadenylated and nonpolyadenylated RNA, beta-tubulin mRNA, and mini-exon-derived RNA (medRNA) was studied. For medRNA, no significant changes in the rate of transcription or stability were observed during differentiation. In contrast, while the rate of transcription of beta-tubulin mRNA in in vitro-cultured bloodstream forms, transforming cells, and established procyclic cells was similar, the half life was four to five times longer in procyclic cells (t1/2, 7 h) than in cultured bloodstream forms (t1/2, 1.4 h) or transforming cells (t1/2, 1.7 h). Inhibition of protein synthesis in bloodstream forms at 37 degrees Celsius caused a dramatic 20-fold decrease in the rate of VSG mRNA synthesis and a 6-fold decrease in half-life to 45 min, while beta-tubulin mRNA was stabilized 2- to 3-fold and mRNA stability remained unaffected. It is postulated that triggering transformation or inhibiting protein synthesis induces changes in the abundance of the same regulatory molecules which effect the shutoff of VSG gene transcription in addition to shortening the half-life of VSG mRNA.

The synthesis and degradation of ras-related gene products during growth and differentiation in Dictyostelium discoideum

A Dictyostelium discoideum protein with an Mr of 23,000 (p23dd-ras) is structurally related to the mammalian proto-oncogene ras-gene product, p21ras, and is specifically precipitated from cell-free extracts of D. discoideum by the Y13-259 monoclonal antibody against p21ras. p23dd-ras was degraded at rates that were very similar to those observed for total protein during both growth and differentiation, suggesting that the previously reported decline in p23dd-ras levels during differentiation is due to a change in the rate of synthesis rather than a change in the rate of degradation. p23dd-ras synthesis did not decrease immediately after the initiation of differentiation, but rather its rate of synthesis increased for the first 1-2 h, suggesting that p23dd-ras is not rapidly down-regulated in response to nutrient deprivation. There were differences in the extent of p23dd-ras turnover during the differentiation of the three tested strains, A-3, NC4, and V12-M2. The relative level of p23dd-ras dropped most rapidly in V12-M2, which may reflect the slightly faster differentiation process exhibited by this strain. In all three strains, very little p23dd-ras was present by the end of the differentiation process. A second protein with an Mr of 24,000 (p24dd-ras) was also immunoprecipitated using the Y13-259 antibody. The amount of p24dd-ras was small or undetectable in vegetative cells, but relatively larger amounts of p24dd-ras were synthesized in pseudoplasmodial cells. We found no evidence to suggest that p24dd-ras is a precursor of p23dd-ras.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of protein synthesis in decay and accumulation of mRNA during spore germination in the cellular slime mold Dictyostelium discoideum

Spore germination in Dictyostelium discoideum is a particularly suitable model for studying the regulation of gene expression, since developmentally regulated changes in both protein and mRNA synthesis occur during the transition from dormant spore to amoeba. The previous isolation of three cDNA clones specific for mRNA developmentally regulated during spore germination allowed for the quantitation of the specific mRNAs during this process. The three mRNAs specific to clones pLK109, pLK229, and pRK270 have half-lives much shorter (minutes) than those of constitutive mRNAs (hours). Using spore germination as a model, we studied the roles of ribosome-mRNA interactions and protein synthesis in mRNA degradation by using antibiotics that inhibit specific reactions in protein biosynthesis. Cycloheximide inhibits the elongation step of protein synthesis. Polysomes accumulate in inhibited cells because ribosomes do not terminate normally and new ribosomes enter the polysome, eventually saturating the mRNA. Pactamycin inhibits initiation, and consequently polysomes break down in the presence of this drug. Under this condition, the mRNA is essentially free of ribosomes. pLK109, pLK229, and pRK270 mRNAs were stabilized in the presence of cycloheximide, but pactamycin had no effect on their normal decay. Since it seems likely that stability of mRNA reflects the availability of sites for inactivation by nucleases, it follows that in the presence of cycloheximide, these sites are protected, presumably by occupancy by ribosomes. No ribosomes are bound to mRNA in the presence of pactamycin, and therefore mRNA degrades at about the normal rate. The data further indicate that a labile protein is probably not involved in mRNA decay or stabilization, since protein synthesis is inhibited equally by both antibiotics. We conclude that it may be important to use more than one type of protein synthesis inhibitor to evaluate whether protein synthesis is required for mRNA decay. The effect of protein synthesis inhibition on mRNA synthesis and accumulation was also studied. mRNA synthesis continues in the presence of inhibitors, albeit at a diminished rate relative to that of the uninhibited control.