Actinomycin D and the regulation of enzyme biosynthesis during development of Dictyostelium discoideum

Live imaging of nascent RNA dynamics reveals distinct types of transcriptional pulse regulation

Transcription of genes can be discontinuous, occurring in pulses or bursts. It is not clear how properties of transcriptional pulses vary between different genes. We compared the pulsing of five housekeeping and five developmentally induced genes by direct imaging of single gene transcriptional events in individual living Dictyostelium cells. Each gene displayed its own transcriptional signature, differing in probability of firing and pulse duration, frequency, and intensity. In contrast to the prevailing view from both prokaryotes and eukaryotes that transcription displays binary behavior, strongly expressed housekeeping genes altered the magnitude of their transcriptional pulses during development. These nonbinary "tunable" responses may be better suited than stochastic switch behavior for housekeeping functions. Analysis of RNA synthesis kinetics using fluorescence recovery after photobleaching implied modulation of housekeeping-gene pulse strength occurs at the level of transcription initiation rather than elongation. In addition, disparities between single cell and population measures of transcript production suggested differences in RNA stability between gene classes. Analysis of stability using RNAseq revealed no major global differences in stability between developmental and housekeeping transcripts, although strongly induced RNAs showed unusually rapid decay, indicating tight regulation of expression.

A new environmentally resistant cell type from Dictyostelium

This paper describes the serendipitous discovery and first characterization of a new resistant cell type from Dictyostelium, for which the name aspidocyte (from aspis: Greek for shield) is proposed. These cells are induced from amoebae by a range of toxins including heavy metals and antibiotics, and were first detected by their striking resistance to detergent lysis. Aspidocytes are separate, rounded or irregular-shaped cells, which are immotile but remain fully viable; once the toxic stress is removed, they revert to amoeboid cells within an hour. Induction takes a few hours and is completely blocked by the protein synthesis inhibitor cycloheximide. Aspidocytes lack a cell wall and their resistance to detergent lysis is active, requiring continued energy metabolism, and may be assisted by a complete cessation of endocytosis, as measured by uptake of the dye FM1-43. Microarray analysis shows that aspidocytes have a distinct pattern of gene expression, with a number of genes up-regulated that are predicted to be involved in lipid metabolism. Aspidocytes were initially detected in a hypersensitive mutant, in which the AMP deaminase gene is disrupted, suggesting that the inductive pathway involves AMP levels or metabolism. Since aspidocytes can also be induced from wild-type cells and are much more resistant than amoebae to a membrane-disrupting antibiotic, it is possible that they are an adaptation allowing Dictyostelium cells to survive a sudden onslaught of toxins in the wild.

Cytokinin oxidase activity in the cellular slime mold, Dictyostelium discoideum

The cytokinin N6-(delta 2-isopentenyl)adenine (i6Ade) is produced during the development of the cellular slime mold, Dictyostelium discoideum, and functions in this organism as the immediate precursor of the spore germination inhibitor, discadenine. The metabolism of i6Ade in axenic cultures of D. discoideum Ax-3 amoebae has been investigated in the present study. An enzyme activity that specifically catalyzes the degradation of i6Ade has been detected in Ax-3 amoebae. This enzyme is similar to the cytokinin oxidases present in higher plant systems and cleaves the N6-side chain of i6Ade to form adenine. Discadenine synthase activity was also detected in axenically cultured Ax-3 amoebae. The cytokinin oxidase activity detected in Dictyostelium decreased during aggregation and development of Ax-3 amoebae and in starving Ax-3 amoebae maintained under either fast-shake (230 rpm) or slow-shake (70 rpm) conditions. In the latter case, the fall in enzyme activity was accelerated by treatment with cyclic AMP. In contrast to these results, discadenine synthase activity in Ax-3 amoebae rose sharply during the culmination phase of development, exhibited little change in starving Ax-3 amoebae maintained under fast-shake conditions, and fell under slow-shake conditions unless the amoebae were treated with cyclic AMP. Possible functions of the Dictyostelium cytokinin oxidase and the significance of the i6Ade metabolism observed in vegetative Dictyostelium amoebae are discussed.

cAMP regulation of early gene expression in signal transduction mutants of Dictyostelium

We have examined the regulation of three early developmentally regulated genes in Dictyostelium. Two of these genes (D2 and M3) are induced by pulses of cAMP and the other (K5) is repressed. Expression of these genes has been examined in a number of developmental mutants that are specifically blocked in various aspects of the signal transduction/cAMP relay system involved in aggregation and control of early development. The mutant strains include Synag mutants, which are blocked in receptor-mediated activation of adenylate cyclase and do not relay cAMP pulses; FrigidA mutants, which are blocked in receptor-mediated activation of both adenylate cyclase and the putative phosphoinositol bisphosphate (PIP2) turnover pathway and appear to be mutations in the gene encoding one of the G alpha protein subunits; and a StreamerF allele, which lacks cGMP-specific cGMP phosphodiesterase. From the analysis of the developmental expression of these genes under a variety of conditions in these mutant strains, we have drawn a number of conclusions concerning the modes of regulation of these genes. Full induction of D2 and M3 genes requires cAMP interaction with the cell surface receptor and an "oscillation" of the receptor between active and adapted forms. Induction of these genes does not require activation of the signal transduction pathway that leads to adenylate cyclase activation and cAMP relay, but does require activation of other receptor-mediated intracellular signal transduction pathways, possibly that involving PIP2 turnover. Likewise, repression of the K5 gene requires pulses of cAMP. Expression of this gene is insensitive to cAMP pulses in FrigidA mutants, suggesting that a signal transduction pathway is necessary for its repression. Results using the StreamerF mutant suggest that the rise in cGMP in response to cAMP/receptor interactions may not be directly related to control of the pulse-induced genes. In addition, we have examined the effect of caffeine, which M. Brenner and S.D. Thomas (1984, Dev. Biol., 101, 136-146) showed preferentially blocks the cAMP relay system by blocking receptor-mediated activation of adenylate cyclase. We show that in many of the mutants and in an axenic wild-type strain, caffeine causes the induction of pulse-induced gene expression to almost wild-type levels or in some cases to higher than wild-type levels. Our data suggest that caffeine works by activating some step in the signal transduction pathway that must lie downstream from both the receptor and at least one of the G proteins and thus has effects other than simply blocking the receptor-mediated cAMP relay system.

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.

Sensitivity of RNA synthesis to actinomycin D inhibition is dependent on the frequency of transcription: a mathematical model

The synthesis of ribosomal RNA is known to be up to 100-fold more sensitive to inhibition by actinomycin D than is messenger RNA. A model is presented here to explain the dose-response kinetics of this inhibition. The basic concept is that on very actively transcribed genes, such as ribosomal DNA, the first bound actinomycin D will sterically affect those densely packed polymerase molecules between it and the promoter, causing them to stack up into the promoter and interfere with the initiation of RNA synthesis. However, on less active genes, where the polymerases are widely spaced, the drug will inhibit individual polymerasas independently and only at the actinomycin blockade. Counteracting these inhibitory effects will be the tendency of genes bound with actinomycin to accumulate additional polymerases. A mathematical model is described which successfully explains previously reported dose-response kinetics of actinomycin inhibition in both frequently and infrequently transcribed genes. The analysis indicates that actinomycin inhibition is dependent on both polymerase packing and on gene size. The dose-response kinetics can be used to estimate both the size and transcriptional efficiency of individual genes. The model is also able to explain several other independent observations regarding the kinetics of inhibition of RNA synthesis by actinomycin.

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.

Primer extension analysis of tRNA gene transcripts synthesized in vitro and in vivo

The primer elongation method has been adapted to analyze tRNA gene transcripts. The primer used to direct cDNA synthesis from a corresponding tRNA template, in the presence of AMV reverse transcriptase, was a restriction fragment, or a synthetic oligonucleotide, containing exclusively coding nucleotides of a tRNA gene. This method not only allows one to identify the exact 5'-end of mature tRNA, but also 5'-ends of primary transcripts are readily determined. Further, analysis of tRNAs synthesized in vitro, as well as tRNAs produced in vivo in homologous and heterologous organisms can be studied. Purification of the tRNAs questioned, from bulk tRNA, is not necessary.

Cyclic AMP regulation of early gene expression in Dictyostelium discoideum: mediation via the cell surface cyclic AMP receptor

We examined two sets of genes expressed early in the developmental cycle of Dictyostelium discoideum that appear to be regulated by cyclic AMP (cAMP). The transcripts of both sets of genes were not detectable in vegetative cells. During normal development on filter pads, RNA complementary to these genes could be detected at about 2 h, peaked around 6 to 8 h, and decreased gradually thereafter. Expression of these genes upon starvation in shaking culture was stimulated by pulsing the cells with nanomolar levels of cAMP, a condition that mimics the in vivo pulsing during normal aggregation. Expression was inhibited by caffeine or by continuous levels of cAMP, a condition found later in development when in vivo expression of these genes decreased. The inhibition of caffeine could be overcome by pulsing cells with cAMP. These results suggest that the expression is mediated via the cell surface cAMP receptor, but does not require a rise in intracellular cAMP. mRNA from a gene of the second class was induced upon starvation, peaked by 2.5 h of development, and then declined. In contrast to the other genes, its expression was maintained by continuous levels of cAMP and repressed by cAMP pulses. These and other results on a number of classes of developmentally regulated genes indicates that changing levels of cAMP, acting via the cell surface cAMP receptor, are involved in controlling these groups of genes. We also examined the structure and partial sequence of the cAMP pulse-induced genes. The two tandemly duplicated M3 genes were almost continuously homologous over the sequenced portion of the protein-coding region except for a region near the N-terminal end. The two M3 genes had regions of homology in the 5' flanking sequence and showed slight homology to the same regions in gene D2, another cAMP pulse-induced gene. D2 showed extremely significant homology over its entire sequenced length to an acetylcholinesterase. The results presented here and by others suggest that expression of many early genes in D. discoideum is regulated via the cell surface cAMP receptor. We expect that many of these genes may play essential roles in early Dictyostelium development and could code for elements of the cAMP signal transduction pathway involved in aggregation.

Cyclic AMP regulation of early gene expression in Dictyostelium discoideum: mediation via the cell surface cyclic AMP receptor

We examined two sets of genes expressed early in the developmental cycle of Dictyostelium discoideum that appear to be regulated by cyclic AMP (cAMP). The transcripts of both sets of genes were not detectable in vegetative cells. During normal development on filter pads, RNA complementary to these genes could be detected at about 2 h, peaked around 6 to 8 h, and decreased gradually thereafter. Expression of these genes upon starvation in shaking culture was stimulated by pulsing the cells with nanomolar levels of cAMP, a condition that mimics the in vivo pulsing during normal aggregation. Expression was inhibited by caffeine or by continuous levels of cAMP, a condition found later in development when in vivo expression of these genes decreased. The inhibition of caffeine could be overcome by pulsing cells with cAMP. These results suggest that the expression is mediated via the cell surface cAMP receptor, but does not require a rise in intracellular cAMP. mRNA from a gene of the second class was induced upon starvation, peaked by 2.5 h of development, and then declined. In contrast to the other genes, its expression was maintained by continuous levels of cAMP and repressed by cAMP pulses. These and other results on a number of classes of developmentally regulated genes indicates that changing levels of cAMP, acting via the cell surface cAMP receptor, are involved in controlling these groups of genes. We also examined the structure and partial sequence of the cAMP pulse-induced genes. The two tandemly duplicated M3 genes were almost continuously homologous over the sequenced portion of the protein-coding region except for a region near the N-terminal end. The two M3 genes had regions of homology in the 5' flanking sequence and showed slight homology to the same regions in gene D2, another cAMP pulse-induced gene. D2 showed extremely significant homology over its entire sequenced length to an acetylcholinesterase. The results presented here and by others suggest that expression of many early genes in D. discoideum is regulated via the cell surface cAMP receptor. We expect that many of these genes may play essential roles in early Dictyostelium development and could code for elements of the cAMP signal transduction pathway involved in aggregation.

Phenocopy of discoidin I-minus mutants by antisense transformation in Dictyostelium

Using an antisense construct of the discoidin gene transfected into Dictyostelium, we have repressed the expression of the three endogenous discoidin genes. Transformants exhibit a greater than 90% reduction in accumulated discoidin mRNA and protein. Nuclear run-on assays show that both the endogenous and the antisense genes are transcribed. Since only minor amounts of endogenous gene transcripts and none from the antisense gene can be detected on blots, we suggest that hybrids are formed within the nucleus and are rapidly degraded. Discoidin is believed to play a role in cell-substratum interaction and exhibits homologies to fibronectin. Discoidin-minus mutants exhibit the developmental phenotype of not streaming on a plastic surface. Antisense transformants show a similar phenotype and are thus phenocopies of these mutants.

Gene-specific expression of the actin multigene family of Dictyostelium discoideum

We have investigated the expression of 14 cloned genes of the 20-member actin multigene family of Dictyostelium discoideum using gene-specific mRNA complementary probes and an RNase protection assay. Actin gene expression was studied in vegetative cells and in cells at a number of developmental stages chosen to represent the known major shifts in actin mRNA and protein synthesis. At least 13 of these genes are expressed. A few genes are expressed very abundantly at 10% or more of total actin mRNA; however, the majority are maximally expressed at 1 to 5% of actin message. Although all of the genes are transcribed in vegetative cells, most genes appear to be independently regulated. Actin 8 appears to be transcribed at constant, high levels throughout growth and development. Actin 12 mRNA is maximally expressed in vegetative cells but the level is reduced appreciably by the earliest stage of development examined, while Actin 7 mRNA is specifically induced approximately sevenfold at this time. The rest of the genes appear to be induced 1.5 to 2-fold early in development, coincident with the increase in total actin mRNA. Since 12 of the genes code for extremely homologous proteins, it is possible that the large number of actin genes in Dictyostelium is utilized for precise regulation of the amount of actin produced at any stage of development, even though individual gene expression appears in some cases to be very stage-specific. In addition to these 13 actin genes, at least two and possibly four more genes are known to be expressed, because they are represented by complementary DNA clones, and an additional one or two expressed genes are indicated by primer extension experiments. Only one known gene, Actin 2-sub 2, is almost certainly a pseudogene. Thus the vast majority of Dictyostelium actin genes are expressed.

Molecular cloning of a cDNA complementary to a UDP-glucose pyrophosphorylase mRNA of dictyostelium discoideum

Uridine diphosphoglucose pyrophosphorylase (UTP: -alpha-D-glucose-1-phosphate uridyltransferase, EC 2.7.7.9) is an essential enzyme for normal development of Dictyostelium discoideum and its specific activity increases 3- to 10-fold by the later stages of development. Previous experiments have shown that additional forms of the enzyme appear concomitantly with this increase and that two uridine diphosphoglucose pyrophosphorylase (UDPGP) polypeptides are immunoprecipitated from the in vitro translation products of total cellular RNA at any stage of development (B. F. Fishel, R. E. Manrow and R. P. Dottin, 1982, Dev. Biol. 92, 175-187). Using an in vitro translation-immunoprecipitation assay of UDPGP mRNA, we show that an increase in the amount of translatable mRNA is correlated with the accumulation of enzyme during development. A cDNA bank was constructed from a mRNA population that had been enriched for UDPGP mRNA by size fractionation on sucrose gradients containing methylmercuric hydroxide (C. W. Schweinfest, R. W. Kwiatkowski, and R. P. Dottin, 1982, Proc. Natl. Acad. Sci. USA 79, 4997-5000). A 1.8-Kb cDNA complementary to a UDPGP mRNA was identified after screening the bank by hybridization selection and translation. Only the mRNA encoding the higher molecular weight in vitro translation product is hybrid selected by this cDNA. In hybrid-arrested translation experiments, the coding strand of this cDNA selectively inhibits the translation of only one of the two in vitro translation products. Therefore, there are two distinct UDPGP mRNAs.

A secreted factor and cyclic AMP jointly regulate cell-type-specific gene expression in Dictyostelium discoideum

We are studying cell differentiation in Dictyostelium discoideum by examining the regulation of genes that are preferentially expressed in different cell types. A system has been established in which prestalk- and prespore-cell-specific genes are expressed in single cells in response to culture conditions. We confirm our previous results showing that cyclic AMP induces prestalk genes and now show that it is also required for prespore gene induction. The expression of both classes of genes is additionally dependent on the presence of a factor(s) secreted by developing cells which we call conditioned medium factor(s). An assay for conditioned medium factor(s) shows that it is detectable within 2.5 h after the onset of development. Conditioned medium factor(s) also promotes the expression of genes induced early in development, but has no detectable effect on the expression of actin genes and a gene expressed maximally in vegetative cells. In the presence of conditioned medium factor(s), exogenous cyclic AMP at the onset of starvation fails to induce the prespore and prestalk genes. The addition of cyclic AMP between 2 and 12 h of starvation results in rapid prestalk gene expression, whereas prespore genes are induced at an invarient time (approximately 18 h after the onset of starvation). These data suggest that cyclic AMP and conditioned medium factor(s) are sufficient for prestalk gene induction, whereas an additional parameter(s) is involved in the control of prespore gene induction. In contrast to several previous studies, we show that multicellularity is not essential for the expression of either prespore or prestalk genes. These data indicate that prespore and prestalk genes have cell-type-specific as well as shared regulatory factors.

Dictyostelium discoideum mRNAs developmentally regulated during spore germination have short half-lives

mRNA decay was studied during spore germination in Dictyoselium discoideum by the use of three previously isolated cDNA clones, pLK109, pLK229, and pRK270, which are specific for mRNAs developmentally regulated during spore germination. The half-life of a constitutive mRNA, pLK125, which is present throughout germination, growth, and development, as also determined. Nogalamycin, a DNA-intercalating compound, was used to inhibit RNA synthesis. Total RNA was isolated at intervals after addition of the drug, and the decay of mRNAs specific for the cDNA clones was determined by both Northern blot and RNA dot hybridization. If nogalamycin was added immediately after activation of dormant spores, neither pLK229 nor pLK109 mRNA decayed, but pLK125 mRNA did decay. Although pLK109 mRNA did not decay under these conditions, the RNA was smaller 1 h after activation than in dormant spores, indicating that it was processed normally. At 1 h after activation, pLK229-, pLK125-specific mRNAs decayed exponentially, with half-lives of 24, 39, and 165 min, respectively. Under the same conditions, decay of pLK109-specific mRNA was biphasic. Thirty-eight percent of the mRNA decayed with a half-life of 5.5 min, and the remainder decayed with a half-life of 115 min. It seems likely that nogalamycin inhibits the synthesis of an unstable component of the mRNA degradative pathway which is needed continuously for the decay of pLK109 mRNA. By extrapolating the curve representing the rapidly decaying component, a half-life of 18 min was calculated for pLK109-specific mRNA. The mRNAs developmentally regulated during spore germination have half-lives shorter than that of the constitutive messenger and shorter than the average half-life of 3 to 4 h previously determined for total Dicyostelium polyadenylated mRNA.

A secreted factor and cyclic AMP jointly regulate cell-type-specific gene expression in Dictyostelium discoideum

We are studying cell differentiation in Dictyostelium discoideum by examining the regulation of genes that are preferentially expressed in different cell types. A system has been established in which prestalk- and prespore-cell-specific genes are expressed in single cells in response to culture conditions. We confirm our previous results showing that cyclic AMP induces prestalk genes and now show that it is also required for prespore gene induction. The expression of both classes of genes is additionally dependent on the presence of a factor(s) secreted by developing cells which we call conditioned medium factor(s). An assay for conditioned medium factor(s) shows that it is detectable within 2.5 h after the onset of development. Conditioned medium factor(s) also promotes the expression of genes induced early in development, but has no detectable effect on the expression of actin genes and a gene expressed maximally in vegetative cells. In the presence of conditioned medium factor(s), exogenous cyclic AMP at the onset of starvation fails to induce the prespore and prestalk genes. The addition of cyclic AMP between 2 and 12 h of starvation results in rapid prestalk gene expression, whereas prespore genes are induced at an invarient time (approximately 18 h after the onset of starvation). These data suggest that cyclic AMP and conditioned medium factor(s) are sufficient for prestalk gene induction, whereas an additional parameter(s) is involved in the control of prespore gene induction. In contrast to several previous studies, we show that multicellularity is not essential for the expression of either prespore or prestalk genes. These data indicate that prespore and prestalk genes have cell-type-specific as well as shared regulatory factors.

Characterisation of a Dictyostelium discoideum DNA fragment coding for a putative tRNAValGUU gene. Evidence for a single transcription unit consisting of two overlapping class III genes

A genomic DNA fragment from Dictyostelium discoideum was characterized. This DNA, although 74% d(A + T)-rich, codes for a putative tRNAValGUU. The tRNAVal gene overlaps at its 5' half with another RNA polymerase III transcription unit. This RNA polymerase III transcription unit can be folded into a tRNA-like shape and is comprised of significant amounts of invariant and semi-invariant nucleotides present in all eukaryotic tRNAs. This unit contains the two promoter blocks defined for RNA polymerase III, which are homologous to recently defined promoter elements to the extent of 76-88% (A block) and 86-93% (B block) respectively [Sharp et al. (1981) Proc. Natl Acad. Sci. USA 78, 6657-6661]. Both of the overlapping class III genes are transcribed in germinal vesicle extracts prepared from Xenopus laevis oocytes as a single transcription unit, resulting in an unusually large product compared to primary transcripts of other tRNA genes. The unit is not transcribed in HeLa extracts but it competes very strongly for transcription factor(s) under the conditions of stable transcription complex formation. Although the whole unit is transcribed, it is believed that only one functional product is formed. Therefore we define the tRNA-like structure, coded for on this class III transcription unit, as a putative tRNA 'pseudogene' meaning that, although it is transcribed by RNA polymerase III, it is not likely to mature to a functional tRNA.

Dictyostelium discoideum mRNAs developmentally regulated during spore germination have short half-lives

mRNA decay was studied during spore germination in Dictyoselium discoideum by the use of three previously isolated cDNA clones, pLK109, pLK229, and pRK270, which are specific for mRNAs developmentally regulated during spore germination. The half-life of a constitutive mRNA, pLK125, which is present throughout germination, growth, and development, as also determined. Nogalamycin, a DNA-intercalating compound, was used to inhibit RNA synthesis. Total RNA was isolated at intervals after addition of the drug, and the decay of mRNAs specific for the cDNA clones was determined by both Northern blot and RNA dot hybridization. If nogalamycin was added immediately after activation of dormant spores, neither pLK229 nor pLK109 mRNA decayed, but pLK125 mRNA did decay. Although pLK109 mRNA did not decay under these conditions, the RNA was smaller 1 h after activation than in dormant spores, indicating that it was processed normally. At 1 h after activation, pLK229-, pLK125-specific mRNAs decayed exponentially, with half-lives of 24, 39, and 165 min, respectively. Under the same conditions, decay of pLK109-specific mRNA was biphasic. Thirty-eight percent of the mRNA decayed with a half-life of 5.5 min, and the remainder decayed with a half-life of 115 min. It seems likely that nogalamycin inhibits the synthesis of an unstable component of the mRNA degradative pathway which is needed continuously for the decay of pLK109 mRNA. By extrapolating the curve representing the rapidly decaying component, a half-life of 18 min was calculated for pLK109-specific mRNA. The mRNAs developmentally regulated during spore germination have half-lives shorter than that of the constitutive messenger and shorter than the average half-life of 3 to 4 h previously determined for total Dicyostelium polyadenylated mRNA.

Protein synthesis during development and differentiation in the cellular slime mould Dictyostelium discoideum

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Comparison of 5'-nucleotidase activities of isolated plasma membranes of two ascites cell variants

The glycogen-containing ascites cell line was found to have a 3-5 times higher 5'-nucleotidase specific activity than the glycogen-free variant, resulting in different substrate affinity constants of Km = 0.14 mM and Km = 0.69 mM respectively. These activity differences were due to true 5'-nucleotidase as shown by its inactivation through specific inhibitors such as concanavalin A and alpha, beta-methylene adenosine diphosphate. Substrate specificity of the enzyme was similar in both cell lines, but differences were observed with respect to the pH optimum and stability.

Induction and modulation of cell-type-specific gene expression in Dictyostelium

We have identified genes that are expressed preferentially in either prestalk or prespore cells in Dictyostelium. The prestalk mRNAs are detectable at 7.5 hr prior to the completion of cell aggregation, while the prespore mRNAs are not detectable until approximately 15 hr of development. Exogenous cAMP in the absence of sustained cell contact is sufficient to induce prestalk-specific gene expression, while multicellularity is required for the induction of prespore-specific genes. A gene expressed equally in both cell types, which has the same developmental kinetics as the prestalk genes, is induced in shaking culture in the absence of either cAMP or stable cell associations. Dissociation of aggregates results in the rapid loss of prespore- and prestalk-specific mRNAs, and these can be induced to reaccumulate with the addition of cAMP. We conclude that there are substantial differences in the timing and requirements for tissue-specific gene expression in Dictyostelium.

Messenger RNA half-life in Dictyostelium discoideum

Messenger RNA half-life in vegetatively growing cells of Dictyostelium discoideum was determined using a uridine pulse-chase procedure. In these experiments, mRNA decayed in a complex fashion, and consisted of at least two major components, one with a half-life of about 50 min and a second with a half-life of about 10 hr. These results independently confirm our previous studies on the decay of mRNA extracted from actinomycin D-treated cells. Since these results were in apparent conflict with half-life determinations obtained with a combination of actinomycin D and daunomycin (J. P. Margolskee and H. F. Lodish, 1980a, Dev. Biol. 74, 37-49), we have also studied mRNA half-life in cells treated with a combination of both drugs and found that simultaneous use of both drugs leads to accelerated mRNA decay and other noticeable side effects. In light of our observations, we have suggested an alternative to conclusions drawn by others with respect to mRNA synthesis and stability in Dictyostelium development.

Nogalamycin inhibits ribonucleic acid synthesis in growing and developing cells of the slime mold Dictyostelium discoideum

Nogalamycin, an anthracycline antibiotic that intercalates into deoxyribonucleic acid, is a potent inhibitor of ribonucleic acid (RNA) synthesis in the slime mold Dictyostelium discoideum. The antibiotic inhibits RNA synthesis in growing cells and in inactivated spores, and in this way inhibits spore germination. Protein synthesis is much less inhibited. Nogalamycin inhibits ribosomal RNA, transfer RNA, and messenger RNA equally. Polysomes break down in the presence of the drug with a half-life of 220 min, and messenger RNA decays with a half-life of 290 min. The data show that nogalamycin can be employed to inhibit messenger RNA synthesis and is useful in determining messenger RNA decay rates in the slime mold.

Cell differentiation and flagellar elongation in Naegleria gruberi. Dependence on transcription and translation

This paper presents evidence that the phenotypic transformation of Naegleria gruberi from amebae to flagellates that occurs when cells are placed in a nutrient-free aqueous environment is dependent on transcription and translation. RNA and protein are synthesized during the hour-long differentiation. Actinomycin D and daunomycin selectively inhibit RNA synthesis, and cycloheximide selectively inhibits protein synthesis, throughout the time required for differentiation. These inhibitors prevent differentiation if added soon after the cells are transferred to nonnutrient buffer but cease to block specific differentiation events at subsequent, reproducible times, the transition points. After each transition point, morphogenesis can occur in the presence of the inhibitor and in the virtual absence of transcription or translation. A map of the transition points indicates that RNA synthesis is required until halfway through the temporal process from initiation to flagellum assembly, and that protein synthesis is required until three-fourths of the way through. Even when flagellum outgrowth can occur in the presence of cycloheximide, the length of the flagella formed is determined by the extent of synthesis of an unknown "limiting precursor." The transition points for formation of flagella and for formation of the streamlined flagellate body shape are temporally separate. These results indicate that differentiation in Naegleria involves a redirection of cell metabolism to produce new RNA and protein molecules that are essential for morphogenesis.

Regulation of induction of phenylalanine ammonia-lyase in suspension cultures of Phaseolus vulgaris

Total RNA was extracted from fast growing suspension cells of bean, the mRNA was translated and the products of protein synthesis analysed by gel electrophoresis. Actinomycin D (20 μg ml(-1)) added to the cultures 12 h before the induction of phenylalanine ammonia-lyase (PAL) activity by naphthylacetic acid (NAA) (1 mg/l) and kinetin (0.2 mg/l) failed to prevent the increased activity of the enzyme usually produced by this ratio of the plant growth hormones. PAL was isolated and purified from suspension cultured bean cells. The purified enzyme ran as a single band on polyacrylamide gel electrophoresis. The protein translated from RNA prepared from induced and non-induced cells was separated by gel electrophoresis and the bands of protein on the gels were compared. There was no evidence for an increase in the amount of PAL synthesised in vitro from the mRNA of induced cells even though these had 5 times the amount of activity of the enzyme compared with that of the non-induced cells. The results indicate that the induction of PAL activity is not immediately preceeded by an increase in the synthesis of PAL-mRNA by the cells. The control of the activity of the enzyme is discussed with respect to this finding.

Induction of cyclic AMP phosphodiesterase by disaggregation of the multicellular complexes of Dictyostelium discoideum

When the multicellular complexes (slugs) of Dictyostelium discoideum were disaggregated, the activity of phosphodiesterase for adenosine 3':5'-monophosphate increased. The increase occurred first with the enzyme in the particulate fraction of the cell. Cytoplasmic and extracellular phosphodiesterase, as well as the macromolecular inhibitor for this enzyme, also increased at later times. The increase in these activities was blocked by inhibitors of protein and RNA synthesis. This indicates that the increase is not due to the activation of the enzyme, but due to its synthesis de novo, and that the synthesis is induced at the transcriptional level by the loss of cell contact. In the case of cellular phosphodiesterase, inhibitors of protein synthesis (cycloheximide and emetine) not only blocked the increase, but also caused a decrease in activity. The mechanism of such decrease was further investigated.

Regulation of macromolecular synthesis during hyphal germ tube emergence from Mucor racemosus sporangiospores

Protein and RNA syntheses were examined during hyphal germ tube emergence from sporangiospores of a dimorphic phycomycete, Mucor racemosus. Both classes of macromolecules were synthesized immediately upon introduction of the dormant sporangiospores into nutrient medium. The specific rates of synthesis of both protein and RNA accelerated during initial germ tube emergence and reached a maximum when the emergence of new germ tubes ended. The specific rates of synthesis later decreased during further hyphal elongation. The distribution of ribosomes between active polysomes and monosomes and inactive subunits was determined by sucrose density gradient centrifugation, and the rate of amino acid addition to nascent polypeptide chains was calculated throughout the developmental sequence. The results showed that both the percentage of ribosomes active in protein synthesis and the velocity of ribosome movement along the mRNA were continuously adjusted throughout hyphal germ tube development. The free intracellular amino acid pools were measured throughout development. Alanine, glutamate, and aspartate were present at very high concentrations in the dormant spores but were rapidly depleted during hyphal germ tube emergence. The results of these studies are discussed in relation to hyphal germ tube development from yeast cells of Mucor and dormant spores of other fungal species.

Messenger RNA for myosin polypeptides: isolation from single myogenic cell cultures

Messenger RNA which stimilates the synthesis of myosin heavy chain in a reticulocyte lysate has been isolated from single myogenic cell cultures. Specific myosin polypeptides have been identified by immunoprecipitation with an antibody made to purified adult chicken skeletal muscle myosin. This mRNA binds to oligo(dT)-cellulose, and an active fraction from sucrose gradients migrates as 26S on formamide-polyacrylamide gels. The relative amount of this RNA increases dramatically at the time of terminal differentiation.

Changes in morphogenesis and developmental enzyme levels in Dictyostelium discoideum after gamma irradiation

Changes in the levels of specific activity of two enzymes believed to be involved in developmental regulation were observed after irradiating differentiating cells of Dictyostelium discoideum. Stimulation of the levels of specific activity of alkaline phosphatase occured after irradiation at the beginning of development and at the end of the aggregation period, but not after irradiation at the beginning of aggregation. A stimulation in UDP-glucose pyrophosphorylase specific activity was also observed, but to a lesser extent and only after irradiation at the end of aggregation. Dose-dependent delays in the appearance of peaks of specific activity were noted. The delay per unit dose was less when irradiation took place at the beginning of development as opposed to the beginning or end of the aggregation period. Radiation-induced delays in progression through visible developmental stages were almost identical to delays in enzyme appearance. Other radiation effects on morphogenesis included the induction of a migratory slug phase.

Growth and differentiation of the water mold Blastocladiella emersonii: cytodifferentiation and the role of ribonucleic acid and protein synthesis

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