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

Evaluation of the Teratogenic Potential of Valproic Acid Analogues in Transgenic Dictyostelium discoideum Strains

Very early during the development of new pharmaceuticals toxicological tests are most important. In addition to acute and chronic toxicity tests, it is crucial to estimate the teratogenic potential of promising drugs. We established a simple biological test system based on the cellular slime mold Dictyostelium discoideum. Under certain environmental conditions single cells of D. discoideum aggregate and undergo a relatively simple cell differentiation program, leading to the formation of stalk and spore cells. Transgenic D. discoideum strains carrying the bacterial beta-galactosidase gene under the control of various developmentally regulated D. discoideum promoters were shown to be useful tools to test the teratogenic potential of valproic acid (VPA). This study describes the effects of the VPA analogues S-4-yn-VPA, R-4-yn-VPA, and 2-ethyl-4-pentynoic acid on the D. discoideum developmental system. The presence of S-4-yn-VPA during D. discoideum development resulted in a strong inhibition of spore cell differentiation, whereas stalk cell formation was less affected. The enantiomer R-4-yn-VPA as well as 2-ethyl-4-pentynoic acid had only moderate effects on D. discoideum development. The above results are consistent with data obtained in mammalian teratogenicity assays, and suggest that D. discoideum development should be investigated with a number of additional substances to provide a simple alternative for high throughput screenings of new drugs.

Transcriptional repression by a bZIP protein regulates Dictyostelium prespore differentiation

In response to the signaling polyketide DIF-1 DimB directly activates transcription of the ecmB gene in pstB cells; a subset of the prestalk cells that are the precursors of the basal disc. We show that the promoter of pspA, a prespore-specific gene, also contains a DimB binding site. Mutation of this site causes ectopic expression in the prestalk region and ChIP analysis shows that DIF-1 induces binding of DimB to the pspA promoter. DIF-1 represses pspA gene expression in a suspension cell assay but this repression is abrogated in a dimB null strain. These results suggest a coupled control mechanism, whereby the same DIF-DimB signaling pathway that directly activates ecmB gene expression directly represses pspA gene expression.

A SET/MYND chromatin re-modelling protein regulates Dictyostelium prespore patterning

SmdA is a Dictyostelium orthologue of the SET/MYND chromatin re-modelling proteins. In developing structures derived from a null mutant for smdA (a smdA- strain), prestalk patterning is normal, but using a prespore lacZ reporter fusion, there is ectopic accumulation of beta-galactosidase in the prestalk region. As wild type slugs migrate, there is continual forward movement and re-differentiation of prespore cells into prestalk cells. Thus, a potential explanation for the ectopic reporter localization in smdA null prestalk cells is an increased rate of re-differentiation and anterior movement of prespore cells. In support of this notion, analysis of an unstable lacZ reporter, driven by the prespore promoter, reveals a normal staining pattern in the smdA- strain. We suggest that one or more genes regulated by SmdA acts to repress prespore re-specification.

Overlapping functions of the two talin homologues in Dictyostelium

Talin is a cytoskeletal protein involved in constructing and regulating focal adhesions in animal cells. The cellular slime mold Dictyostelium discoideum has two talin homologues, talA and talB, and earlier studies have characterized the single knockout mutants. talA(-) cells show reduced adhesion to the substrates and slightly impaired cytokinesis leading to a high proportion of multinucleated cells in the vegetative stage, while the development is normal. In contrast, talB(-) cells are characterized by reduced motility in the developmental stage, and they are arrested at the tight-mound stage. Here, we created and analyzed a double mutant with a disruption of both talA and talB. Defects in adhesion to the substrates, cytokinesis, and development were more severe in cells with a disruption of both talA and talB. The talA(-) talB(-) cells failed to attach to the substrates in the vegetative stage, exhibited a higher proportion of multinucleated cells than talA(-) cells, and showed more-reduced motility during the development and an earlier developmental arrest than talB(-) cells at the loose-mound stage. Moreover, overexpression of either talA or talB compensated for the loss of the other talin, respectively. The analysis of talA(-) talB(-) cells also revealed that talin was required for the formation of paxillin-rich adhesion sites and that there was another adhesion mechanism which is independent of talin in the developmental stage. This is the first study demonstrating overlapping functions of two talin homologues, and our data further indicate the importance of talin.

CUL-4A stimulates ubiquitylation and degradation of the HOXA9 homeodomain protein

The HOXA9 homeodomain protein is a key regulator of hematopoiesis and embryonic development. HOXA9 is expressed in primitive hematopoietic cells, and its prompt downregulation is associated with myelocytic maturation. Although transcriptional inactivation of HOXA9 during hematopoietic differentiation has been established, little is known about the biochemical mechanisms underlying the subsequent removal of HOXA9 protein. Here we report that the CUL-4A ubiquitylation machinery controls the stability of HOXA9 by promoting its ubiquitylation and proteasome-dependent degradation. The homeodomain of HOXA9 is responsible for CUL-4A-mediated degradation. Interfering CUL-4A biosynthesis by ectopic expression or by RNA-mediated interference resulted in alterations of the steady-state levels of HOXA9, mirrored by impairment of the ability of 32D myeloid progenitor cells to undergo proper terminal differentiation into granulocytes. These results revealed a novel regulatory mechanism of hematopoiesis by ubiquitin-dependent proteolysis.

Tyrosine phosphorylation-independent nuclear translocation of a dictyostelium STAT in response to DIF signaling

We describe a Dictyostelium STAT, Dd-STATc, which regulates the speed of early development and the timing of terminal differentiation. Dd-STATc also functions as a repressor, which directs graded expression of the ecmA gene in different prestalk cell populations. Developing Dictyostelium cells produce a chlorinated hexaphenone, DIF, which directs prestalk cell differentiation. Dd-STATc is tyrosine phosphorylated, dimerizes, and translocates to the nucleus when cells are exposed to DIF. Surprisingly, however, SH2 domain-phosphotyrosine interaction is not necessary for the DIF-induced nuclear translocation of Dd-STATc. In this respect, Dd-STATc activation resembles several recently described, noncanonical mammalian STAT signaling processes. We show instead that DIF mediates nuclear translocation via sequences located in the divergent, N-terminal half of the Dd-STATc molecule.

Inducible nuclear translocation of a STAT protein in Dictyostelium prespore cells: implications for morphogenesis and cell-type regulation

Dd-STATa, the Dictyostelium STAT (signal transducer and activator of transcription) protein, is selectively localised in the nuclei of a small subset of prestalk cells located in the slug tip. Injection of cAMP into the extracellular spaces in the rear of the slug induces rapid nuclear translocation of a Dd-GFP:STATa fusion protein in prespore cells surrounding the site of injection. This suggests that cAMP signals that emanate from the tip direct the localised nuclear accumulation of Dd-STATa. It also shows that prespore cells are competent to respond to cAMP, by Dd-STATa activation, and it implies that cAMP signalling is in some way limiting in the rear of the slug. Co-injection of a specific inhibitor of the cAR1 serpentine cAMP receptor almost completely prevents the cAMP-induced nuclear translocation, showing that most or all of the cAMP signal is transduced by cAR1. Dd-GFP:STATa also rapidly translocates into the nuclei of cells adjoining the front and back cut edges when a slug is bisected. Less severe mechanical disturbances, such as pricking the rear of a slug with an unfilled micropipette, also cause a more limited nuclear translocation of Dd-GFP:STATa. We propose that these signalling events form part of a repair mechanism that is activated when the migrating slug suffers mechanical damage.

Identification of cis-regulating elements and trans-acting factors regulating the expression of the gene encoding the small subunit of ribonucleotide reductase in Dictyostelium discoideum

We have examined the promoter of rnrB, the gene encoding the small subunit of ribonucleotide reductase of Dictyostelium discoideum, using lacZ as a reporter gene. Deletion analysis showed that expression of this gene in vegetative cells involves an A/T-rich element, whereas its expression in prespore cells during development requires a region encompassing two G/C-rich elements, designated box A and box B. Removal of boxes A and B results in very low level of activity. When either box A or box B is deleted, prestalk cells adjacent to the prespore zone also express beta-galactosidase. The behavior of these cis-regulatory elements implies that the mechanism regulating the prespore-specific expression of rnrB is different from that regulating other known prespore genes. We have used electrophoretic mobility shift assays to identify factors that interact with box A and box B. Box A interacts with a factor that is found in the nuclear fraction. While box B interacts with a factor that is present in the cytosolic fraction throughout growth and development, its presence in the nuclear fraction is developmentally regulated. Results from competition assays suggest that both box A and box B interact with transcriptional activators that have not been characterized previously.

Cell-type-specific rescue of myosin function during Dictyostelium development defines two distinct cell movements required for culmination

Mutant Dictyostelium cells lacking any of the component polypeptides of myosin II exhibit developmental defects. To define myosin's role in establishing Dictyostelium's developmental pattern, we have rescued myosin function in a myosin regulatory light chain null mutant (mlcR-) using cell-type-specific promoters. While mlcR- cells fail to progress beyond the mound stage, expression of RLC from the prestalk promoter, ecmA, produces culminants with normal stalks but with defects in spore cell localization. When GFP-marked prestalk and prespore cells expressing ecmA-RLC are mixed with wild-type cells, the mislocalization of prestalk cells, but not prespore cells, is rescued. Time-lapse video recording of ecmA-RLC cells showed that the posterior prespore zone failed to undergo a contraction important for the upward movement of prespore cells. Prespore cells marked with green fluorescent protein (GFP) failed to move toward the tip with the spiral motion typical of wild type. In contrast, expression of RLC in prespore cells using the psA promoter produced balloon-like structures reminiscent of sorocarps but lacking stalks. GFP-labeled prespore cells showed a spiral movement toward the top of the structures. Expression of RLC from the psA promoter restores the normal localization of psA-GFP cells, but not ecmA-GFP cells. These results define two distinct, myosin-dependent movements that are required for establishing a Dictyostelium fruiting body: stalk extension and active movement of the prespore zone that ensures proper placement of the spores atop the stalk. The approach used in these studies provides a direct means of testing the role of cell motility in distinct cell types during a morphogenetic program.

Evidence that the RdeA protein is a component of a multistep phosphorelay modulating rate of development in Dictyostelium

We have isolated an insertional mutant of Dictyostelium discoideum that aggregated rapidly and formed spores and stalk cells within 14 h of development instead of the normal 24 h. We have shown by parasexual genetics that the insertion is in the rdeA locus and have cloned the gene. It encodes a predicted 28 kDa protein (RdeA) that is enriched in charged residues and is very hydrophilic. Constructs with the DNA for the c-Myc epitope or for the green fluorescent protein indicate that RdeA is not compartmentalized. RdeA displays homology around a histidine residue at amino acid 65 with members of the H2 module family of phosphotransferases that participate in multistep phosphoryl relays. Replacement of this histidine rendered the protein inactive. The mutant is complemented by transformation with the Ypd1 gene of Saccharomyces cerevisiae, itself an H2 module protein. We propose that RdeA is part of a multistep phosphorelay system that modulates the rate of development.

Transcriptional analysis of the EhPgp1 promoter of Entamoeba histolytica multidrug-resistant mutant

We present here the cloning and characterization of the EhPgp1 multidrug resistance gene promoter isolated from the Entamoeba histolytica drug-resistant mutant clone C2. The EhPgp1 promoter lacks the typical TATA box and the transcriptional initiation sequences described for other E. histolytica promoters. The major transcription initiation site of the EhPgp1 gene was located at the ATG start codon. The EhPgp1 core promoter located within the first 244 base pairs showed a higher chloramphenicol acetyltransferase expression in the transfected trophozoites of clone C2 than in those of the sensitive clone A. Gel shift assays revealed three specific DNA-protein complexes (Ia, IIa, and IIIc) using nuclear extracts from clone C2, whereas three main complexes (If, IIf, and IIg) were limited to clone A. Competition assays suggested the presence of C/EBP-like and OCT-like proteins in complexes Ia and IIa, respectively, probably involved in the expression of the EhPgp1 gene, whereas complex IIIc was competed by GATA-1, C/EBP, OCT, and HOX oligonucleotides. Thus, differential DNA-protein complexes may be formed by transcriptional factors involved in the regulation of the EhPgp1 gene expression.

UGUS, a reporter for use with destabilizing N-termini

Due to constraints in vector construction, reporter polypeptides often carry N-terminal sequences of extraneous origin. Since protein half-life can be influenced by small determinants in the N-terminus, such foreign sequences can destabilize proteins and compromise results of reporter-based studies. We provide a real-life example of this problem (destabilizing sequences derived from a ribosomal protein) and show that it can be solved with the ubiquitin fusion technique, in which ubiquitin sequences are placed upstream of the reporter, in our case beta-glucuronidase. Post-translational processing by characterized pathways removes the ubiquitin together with destabilizing sequences, generating a stable reporter whose N-terminus is constant.

Transcription initiation is controlled by three core promoter elements in the hgl5 gene of the protozoan parasite Entamoeba histolytica

Entamoeba histolytica is a single cell eukaryote that is the etiologic agent of amoebic colitis. Core promoter elements of E. histolytica protein encoding genes include a TATA-like sequence (GTATTTAAAG/C) at -30, a novel element designated GAAC (GAACT) that has a variable location between TATA and the site of transcription initiation, and a putative initiator (Inr) element (AAAAATTCA) overlying the site of transcription initiation. The presence of three separate conserved sequences in a eukaryotic core promoter is unprecedented and prompted examination of their roles in regulating transcription initiation. Alterations of all three regions in the hgl5 gene decreased reporter gene activity with the greatest effect seen by mutation of the GAAC element. Positional analysis of the TATA box demonstrated that transcription initiated consistently 30-31 bases downstream of the TATA region. Mutation of either the TATA or GAAC elements resulted in the appearance of new transcription start sites upstream of +1 in the promoter of the hgl5 gene. Mutation of the Inr element resulted in no change in the site of transcription initiation; however, in the presence of a mutated TATA and GAAC regions, the Inr element controlled the site of transcription initiation. We conclude that all three elements play a role in determining the site of transcription initiation. The variable position of the GAAC element relative to the site of transcription initiation, and the multiple transcription initiations that resulted from its mutation, indicate that the GAAC element has an important and apparently novel role in transcriptional control in E. histolytica.

Evolution of promoter sequences: elements of a canonical promoter for prespore genes of Dictyostelium

An attempt is made to define a minimal prespore promoter which contains all elements essential for correct regulation of expression of a prespore gene. The prespore genes of Dictyostelium are coregulated during development. Most begin transcription at the same early stage, and activity of all is restricted to prespore tissue during the later slug stage. Sequences 5' to the coding sequences of eight prespore genes were searched for all elements proposed to control transcription and for new elements. The meaningfulness of occurrences of elements and pairs of elements in prespore promoters was evaluated by comparison with frequencies of occurrences in promoters of other, nonprespore genes. These comparisons resulted in definition of a canonical prespore promoter, a stretch of about 200 nucleotides containing at least one of each of three elements. Certain limitations were found on the spacing of elements. Orientation of elements with respect to each other appeared unrestricted. All elements often occurred in multiple copies. This structure suggests that individual copies of each element are not conserved during evolution, but instead continually appear and disappear.

Upstream regulatory elements controlling expression of the Entamoeba histolytica lectin

Entamoeba histolytica genomic organization and putative promoter elements appear to be distinct from both metazoan and better characterized protozoan organisms. The recent development of DNA-mediated transfection for E. histolytica enabled characterization of cis-acting promoter elements required for gene expression. A deletion and replacement analysis was conducted on the promoter of an E. histolytica gene encoding the heavy subunit of the N-acetyl-beta-D-galactosamine-specific adhesin (hgl5). Deletion of the DNA from -1000 bases to -272 bases upstream from the start of transcription of hgl5 did not decrease reporter gene expression. Subsequent nested deletions and 10-bp replacement mutagenesis identified four positive upstream regulatory elements between bases -219 to -200, -189 to -160, -69 to -60, and -49 to -40. A negative upstream regulatory element between bases -89 to -80 was conserved upstream of three other E. histolytica genes. Mutation of the previously unidentified 'GAAC' element conserved within the putative core promoter decreased reporter gene expression by 75%. Site directed mutagenesis of the putative TATA element decreased reporter gene expression by greater than 50%, while mutation of the putative initiator element resulted in a more modest decrease. This analysis suggests that E. histolytica promoters are unlike other protozoan promoters, with AT-rich upstream regulatory elements, a non-consensus TATA element, the "GAAC' element, and an unusual initiator element.

Evidence for positional differentiation of prestalk cells and for a morphogenetic gradient in Dictyostelium

We present evidence that Dictyostelium slug tip cells, the pstA cells, may arise by positional differentiation, but at a site remote from that which they will eventually occupy. When first detectable, the pstA cells form a peripheral ring surrounding the other prestalk cell subtype, the pstO cells, but subsequently move above the pstO cells to form the tip. Because pstA cell differentiation requires a 10-fold higher concentration of differentiation-inducing factor, the stalk cell inducer, the initial patterning seems likely to reflect the existence of a morphogenetic gradient. The subsequent redistribution of the two cell types is explicable by their different rates of chemotaxis to cyclic AMP. These results help reconcile the two apparently opposing views of pattern formation in Dictyostelium, that there is positional differentiation and that pattern formation occurs by cell sorting.

The ‘prespore-like cells’ of Dictyostelium have ceased to express a prespore gene: analysis using short-lived beta-galactosidases as reporters

In transgenic strains of Dictyostelium discoideum that express beta-galactosidase under the control of a prespore-specific promoter, only early slugs show reporter confined to the prespore zone. As slugs migrate beta-galactosidase-positive cells accumulate in the prestalk zone; ultimately, there may be so many that the prestalk-prespore boundary is no longer distinguishable (Harwood, A., Early, A., Jermyn, K. and Williams, J. (1991) Differentiation 46, 7–13). It is not clear whether these ‘anomalous’ reporter-positive cells currently express prespore genes; another possibility is that they are ex-prespore cells that have transformed to prestalk and sorted to the prestalk zone (Sternfeld, J. (1993) Roux Archiv. Dev. Biol. 201, 354–363), while retaining their previously produced reporter. To test the activity of the prespore genes in these cells, we have made prespore reporter constructs whose products decay quickly; these are based on constructs used to investigate protein turnover in yeast (Bachmair, A., Finley, D. and Varshavsky, A. (1986) Science 234, 179–186). In strains bearing such constructs, beta-galactosidase-positive cells do not appear in the prestalk zone. The apparent deterioration of the prestalk/prespore pattern in older slugs is thus an artefact of reporter stability.

A GBF-binding site and a novel AT element define the minimal sequences sufficient to direct prespore-specific expression in Dictyostelium discoideum

In order to better understand the molecular mechanisms of cellular differentiation in Dictyostelium discoideum, we have identified the minimum regulatory sequences of the prespore-specific gene SP60/cotC that are sufficient to confer cell-type-specific expression on a heterologous promoter. This region includes at least two essential cis-acting elements: a novel AT-rich element (or elements) and CAE3. The essential function of the AT element is confirmed through point mutations that decrease expression below the level of detection. CAE3 is one of three CA-rich elements (CAEs) required for the induction of SP60/cotC during development or in response to extracellular cyclic AMP. The CAEs have differential affinities for a specific developmentally induced nuclear activity (CAE1 > CAE2 >> CAE3). Here, we identify this activity as G-box-binding factor (GBF) and show that in vitro-transcribed and -translated GBF binds all three SP60/cotC CAEs in a sequence-specific manner. Previous studies have suggested that GBF mediates the induction of some prestalk genes, and these results demonstrate that it also has a specific role in prespore gene activation.

A GBF-binding site and a novel AT element define the minimal sequences sufficient to direct prespore-specific expression in Dictyostelium discoideum

In order to better understand the molecular mechanisms of cellular differentiation in Dictyostelium discoideum, we have identified the minimum regulatory sequences of the prespore-specific gene SP60/cotC that are sufficient to confer cell-type-specific expression on a heterologous promoter. This region includes at least two essential cis-acting elements: a novel AT-rich element (or elements) and CAE3. The essential function of the AT element is confirmed through point mutations that decrease expression below the level of detection. CAE3 is one of three CA-rich elements (CAEs) required for the induction of SP60/cotC during development or in response to extracellular cyclic AMP. The CAEs have differential affinities for a specific developmentally induced nuclear activity (CAE1 > CAE2 >> CAE3). Here, we identify this activity as G-box-binding factor (GBF) and show that in vitro-transcribed and -translated GBF binds all three SP60/cotC CAEs in a sequence-specific manner. Previous studies have suggested that GBF mediates the induction of some prestalk genes, and these results demonstrate that it also has a specific role in prespore gene activation.

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.

Patterns of cell movement within the Dictyostelium slug revealed by cell type-specific, surface labeling of living cells

There are cells scattered in the rear, prespore region of the Dictyostelium slug that share many of the properties of the prestalk cells and that are therefore called anterior-like cells (ALCs). By placing the gene encoding a cell surface protein under the control of an ALC-specific promoter and immunologically labeling the living cells, we analyze the movement of ALCs within the slug. There is a posterior to anterior cellular flow, and the ALCs change their movement pattern as they enter the prestalk zone. Prestalk cells are periodically shed from the migrating slug. They must be replaced if the correct ratio of prestalk to prespore cells is to be maintained, and we present evidence for the transdifferentiation of prespore into prestalk cells, with ALCs functioning as intermediates in the transition. The slug has, therefore, a surprisingly dynamic structure, both with respect to cellular differentiation and cell movement.

Characterization of a novel Dictyostelium discoideum prespore-specific gene, PspB, reveals conserved regulatory sequences

While Dictyostelium discoideum has been studied as a developmental system for decades, and many regulatory proteins have been cloned, the molecular mechanisms of cell-type-specific gene expression are poorly understood. In this paper we characterize a novel prespore gene, PspB, and undertake a comparative analysis of the regulatory regions in prespore-specific D. discoideum promoters. Sequence alignment of the PSPB gene product with other prespore-specific proteins identifies a conserved, repeated 12 amino acid cysteine-containing motif that may be involved in spore coat function or assembly. Analysis of the PspB promoter identifies two domains essential for developmentally induced promoter activity. The first region includes two CA-rich elements (CAEs) that we show to be functionally homologous to the cAMP-inducible elements previously identified in the SP60 (cotC) promoter. The PspB CAEs compete with the SP60 (cotC) CAEs for binding in vitro to a developmentally regulated nuclear activity. We identify this activity as G-box Binding Factor, a developmentally induced transcription factor. The PspB CAEs and adjacent nucleotides direct a very low level of prespore-enriched expression, but high levels of cell-type-specific expression requires a second promoter region: a 46-bp AT-rich sequence that does not resemble the CAEs or any other previously described late gene promoter elements. Comparison of the PspB AT element with regulatory regions of the SP60 (cotC), SP70 (cotB), and D19 (pspA) promoters reveals an extensive consensus sequence. We suggest that these AT-rich sequences may represent a common regulatory element (or elements) required for prespore gene activation.

Cooperation of positively and negatively acting promoter elements determines prespore-specific transcription of Dp87 gene in Dictyostelium

Dp87 gene in Dictyostelium is a novel prespore-specific gene, whose expression is first observed when the aggregation stream is formed, the earliest among prespore-specific genes so far isolated. By 5'-sequential deletion analyses, we had previously indicated that the region between -447 and -356 is important for transcription. Here we show by detailed analyses that the regulatory mechanism of the gene is more complex in that multiple positive and negative regulatory regions including the previously identified region act cooperatively. In addition, we show that the region including the putative TATA box and the transcriptional start site is required for proper negative regulation of the gene.

Redundant regulatory elements account for the developmental control of a ribosomal protein gene of Dictyostelium discoideum

In Dictyostelium discoideum, ribosomal protein genes along with other growth specific genes appear to be coordinately regulated, primarily in response to differences in the translational capacity of developing versus growing cells. In particular, expression of the members of this large class of genes is rapidly and dramatically deactivated when the developmental program is initiated and growth and division cease. In order to understand the mechanisms behind the deactivation event and how it is coupled to the transition from growth to development, we have analyzed the promoter of the V18 gene, a ribosomal protein gene characteristic of this class of growth specific genes. We have delineated three discrete regions involved in the transcription and regulation of the V18 gene. A initiator region which appears to function in a TATA-independent manner was required for transcription and for establishing start site utilization. Two regions upstream of this were defined, both of which were found to independently confer proper developmental regulation.

Developmental regulation of transcription of a novel prespore-specific gene (Dp87) in Dictyostelium discoideum

The Dp87 is a novel prespore specific gene of Dictyostelium discoideum which has a long open reading frame of 555 amino acids. The entire amino acid sequence had low but significant homology to the spore coat proteins, SP96 and SP70, of this organism. When a chimeric gene, containing a 1380 bp of the 5′ upstream region of this gene fused with CAT gene, as reporter, was introduced into cells of this organism, it was expressed only in prespore cells of the slug. Transformation experiments, using chimeric genes, containing a series of 5′ deletions of the upstream region, showed that −447 bp to −357 bp is an important cis-acting regulatory region for transcription. A nuclear factor(s) that specifically bind to this cis-acting region were detected from slug cell nuclei. Transformation experiments using a chimeric gene consisting of the 5′ region between −666 bp and +149 bp of this gene, a beta-galactosidase reporter and an actin 8 terminator, showed that the reporter gene was expressed as early as in aggregation streams, indicating that Dp87 become transcribed a few hours earlier than the other prespore-specific genes so far reported. This was confirmed by northern hybridization detected using an image plate analyzer. The fact that cells expressing Dp87 appeared at random in aggregation streams gives solid support to the idea that position-independent differentiation of prespore and prestalk cells, followed by their sorting, brings about pattern formation in this organism.

Inducible expression of calmodulin antisense RNA in Dictyostelium cells inhibits the completion of cytokinesis

The single gene encoding calmodulin in the eukaryotic microorganism Dictyostelium discoideum was cloned and sequenced. The gene was found to contain three introns, one lying immediately after the translation initiation codon. The deduced amino acid sequence indicated that Dictyostelium calmodulin contains 19 amino acid differences from vertebrate calmodulin, including extensions at both termini. Northern blot analysis showed that similar levels of calmodulin mRNA are present throughout growth and development of wild-type cells. A complete copy of the calmodulin cDNA was prepared, and an 87-base pair fragment complementary to the 5'-end of the calmodulin mRNA was subcloned into the Dictyostelium transformation vector pVEII, such that expression of the antisense transcript was driven by the discoidin I gamma promoter. Transformed cells were selected and maintained at low cell density, a condition resulting in minimal activity of the discoidin I promoter. High level expression was induced by allowing the transformants to reach high cell density or by growing them in the presence of medium conditioned by high density cells. Under these conditions, in which calmodulin mRNA and protein levels were reduced about twofold, the calmodulin antisense transformants lost the ability to complete cytokinesis. A contractile ring formed and constricted, but the midbody linking daughter cells failed to break. The resulting cell population contained multinucleated cells and networks of cells connected by cytoplasmic bridges. Normal cell division was restored when the cells were diluted to low density. These observations have identified a new point at which calmodulin may regulate cell cleavage.

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.

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.

Structure, expression, and regulation of members of the developmentally controlled V and H gene classes from Dictyostelium

We have examined the expression and structure of vegetative specific genes belonging to the V and H gene classes. Both classes of genes are deactivated at the onset of development by a reduction in the rate of transcription. Thus, the genes must be reactivated when the terminally differentiated spores germinate and the resulting amebae return to the vegetative state. During germination, activation of expression of most members of the V gene class was found to parallel the emergence of amoebae from the spore coats. The activation of the V genes did not occur when protein synthesis was inhibited. The timing of activation of the H genes was more heterogeneous and did not parallel emergence. H gene activation occurred even when protein synthesis was inhibited. V4 was found to be the only vegetative specific gene that was responsive to the presence of bacteria. V4 expression was induced by 25-100 fold via transcriptional activation when bacteria were added to amebae growing axenically. Isolation and sequence analysis of the corresponding genomic clones revealed that two V genes, V18 and V1, encode ribosomal proteins. Promoter analysis has delineated the sequences necessary for expression and regulation for several of the V and H genes. In all cases, expression was determined by sequences within the first several hundred base pairs of the transcription start site. For V18 and V14, a positive constitutive element was identified in addition to the sequences involved in regulation. Finally, all of the characterizations and findings are discussed in terms of postulated models for V and H gene expression and regulation.

Coordinate regulation of the spore coat genes in Dictyostelium discoideum

Genomic clones of the genes coding for the three major spore coat proteins, SP60, SP70, and SP96, were used to measure the accumulation of their respective mRNAs in mutant and wild-type cells allowed to develop under a variety of conditions. These prespore-specific mRNAs were found to be both temporally and quantitatively coordinate under all conditions indicating that they may be subject to identical regulatory processes. Accumulation of the spore coat mRNAs is dependent upon the function of both cAMP receptors and G alpha 2 proteins during the aggregation stage as well as upon concomitant protein synthesis. When cells are dissociated from aggregates at 10 hr of development and rapidly shaken in 0.1 mM EDTA they form clumps but do not accumulate any of the prespore-specific RNAs assayed. However, if either 0.1 mM Ca++ or 20 microM cAMP is added to these cells, the spore coat mRNAs accumulate. Lower concentrations of either Ca++ or cAMP had no effect. These results suggest that expression of the spore coat genes normally involves a Ca+(+)-dependent process, but the Ca++ requirement can be overcome by adding high concentrations of exogenous cAMP. Addition of 50 nM DIF to dissociated cell blocks the accumulation of the spore coat mRNAs even when cAMP or Ca++ is present. The upstream regions of the spore coat genes were compared to those of another gene, D19, that codes for the prespore-specific protein SP29. Short sequences related to CACCCAC were found at about the same position relative to the transcriptional start sites of these coordinately regulated genes.

Isolation and characterization of spore coat protein (sp96) gene of Dictyostelium discoideum

A cDNA library was constructed from poly(A)+ RNA isolated from slug cells of Dictyostelium discoideum, using lambda gt11 phage, and screened with an antiserum specific for the spore coat protein sp96. A positive clone was obtained and the gene product was identified as sp96. The sp96 mRNA is 2.2 kb in size, and it starts to accumulate at the tipped aggregate stage only in prespore cells. Southern analysis using nuclear DNA established that the sp96 gene is unique. Two genomic clones containing the sp96 gene were isolated and the sequence of the gene established. The coding region contains a long open reading frame interrupted by a single intron.

Developmental expression and characterization of the gene encoding spore coat protein SP60 in Dictyostelium discoideum

The complete coding sequence, upstream sequence and developmental expression of Dictyostelium discoideum AX2 spore coat protein gene SP60 is reported. The gene contains two exons, 154bp and 1121bp long, separated by a 119bp intron, and encodes a protein of 46,925 molecular weight plus a 23-amino-acid hydrophobic leader sequence. The N-terminus of the mature protein consists of four copies of a perfect hexapeptide repeat (GDWNNN). The central region is rich in cysteine residues, including four highly conserved cysteine-rich repeats with homology to 'EGF-like' repeats. The C-terminus is aspartate-rich and composed of multiple imperfect copies of a D(G/D)DYD repeat followed by several repeats of the tetrapeptide DNDW and derived sequences. A TATA box promoter motif juxtaposed to an oligo(dA) stretch lies 52bp upstream of the main transcriptional start site of the gene. Six AC-rich boxes occur in the region -327 to -556, all of which contain the consensus sequence CACAC. Two GC-rich boxes and a C-rich element (TTACCCCA) are also present upstream. Another open reading frame is positioned a short distance downstream of the SP60 gene in the opposite transcriptional orientation. Expression of the SP60 gene ceases upon disaggregation to single cells and cannot be restored by high levels of extracellular cAMP either alone or in combination with conditioned medium factors.

Optimization and in situ detection of Escherichia coli beta-galactosidase gene expression in Dictyostelium discoideum

We show that a fusion gene, containing the promoter and 5'-noncoding region of a Dictyostelium discoideum actin 6 gene linked to the Escherichia coli beta-galactosidase (beta Gal) gene (lacZ), directs the production of functionally active beta Gal in D. discoideum and that the enzyme can be detected by staining in situ; a procedure which will be of great value in analyzing cell-type-specific gene expression. We illustrate this by fusing lacZ to the promoter of the prespore-specific gene, D19, and localizing expressing cells in migrating slugs. Optimal expression requires the inclusion of termination and polyadenylylation signals and we describe pDDlac, a vector containing a multiple cloning site upstream from a lacZ-Dictyostelium terminator fusion, which can be used to analyze regulated promoters.