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

Genetic control of morphogenesis in Dictyostelium

Cells grow, move, expand, shrink and die in the process of generating the characteristic shapes of organisms. Although the structures generated during development of the social amoeba Dictyostelium discoideum look nothing like the structures seen in metazoan embryogenesis, some of the morphogenetic processes used in their making are surprisingly similar. Recent advances in understanding the molecular basis for directed cell migration, cell type specific sorting, differential adhesion, secretion of matrix components, pattern formation, regulation and terminal differentiation are reviewed. Genes involved in Dictyostelium aggregation, slug formation, and culmination of fruiting bodies are discussed.

A cysteine-rich extracellular protein containing a PA14 domain mediates quorum sensing in Dictyostelium discoideum

Much remains to be understood about quorum-sensing factors that allow cells to sense their local density. Dictyostelium discoideum is a simple eukaryote that grows as single-celled amoebae and switches to multicellular development when food becomes limited. As the growing cells reach a high density, they begin expressing discoidin genes. The cells secrete an unknown factor, and at high cell densities the concomitant high levels of the factor induce discoidin expression. We report here the enrichment of discoidin-inducing complex (DIC), an approximately 400-kDa protein complex that induces discoidin expression during growth and development. Two proteins in the DIC preparation, DicA1 and DicB, were identified by sequencing proteolytic digests. DicA1 and DicB were expressed in Escherichia coli and tested for their ability to induce discoidin during growth and development. Recombinant DicB was unable to induce discoidin expression, while recombinant DicA1 was able to induce discoidin expression. This suggests that DicA1 is an active component of DIC and indicates that posttranslational modification is dispensable for activity. DicA1 mRNA is expressed in vegetative and developing cells. The mature secreted form of DicA1 has a molecular mass of 80 kDa and has a 24-amino-acid cysteine-rich repeat that is similar to repeats in Dictyostelium proteins, such as the extracellular matrix protein ecmB/PstA, the prespore cell-inducing factor PSI, and the cyclic AMP phosphodiesterase inhibitor PDI. Together, the data suggest that DicA1 is a component of a secreted quorum-sensing signal regulating discoidin gene expression during Dictyostelium growth and development.

LrrA, a novel leucine-rich repeat protein involved in cytoskeleton remodeling, is required for multicellular morphogenesis in Dictyostelium discoideum

Cell sorting by differential cell adhesion and movement is a fundamental process in multicellular morphogenesis. We have identified a Dictyostelium discoideum gene encoding a novel protein, LrrA, which composes almost entirely leucine-rich repeats (LRRs) including a putative leucine zipper motif. Transcription of lrrA appeared to be developmentally regulated with robust expression during vegetative growth and early development. lrrA null cells generated by homologous recombination aggregated to form loose mounds, but subsequent morphogenesis was blocked without formation of the apical tip. The cells adhered poorly to a substratum and did not form tight cell-cell agglomerates in suspension; in addition, they were unable to polarize and exhibit chemotactic movement in the submerged aggregation and Dunn chamber chemotaxis assays. Fluorescence-conjugated phalloidin staining revealed that both vegetative and aggregation competent lrrA(-) cells contained numerous F-actin-enriched microspikes around the periphery of cells. Quantitative analysis of the fluorescence-stained F-actin showed that lrrA(-) cells exhibited a dramatically increase in F-actin as compared to the wild-type cells. When developed together with wild-type cells, lrrA(-) cells were unable to move to the apical tip and sorted preferentially to the rear and lower cup regions. These results indicate that LrrA involves in cytoskeleton remodeling, which is needed for normal chemotactic aggregation and efficient cell sorting during multicellular morphogenesis, particularly in the formation of apical tip.

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.

Dictyostelium amoebae lacking an F-box protein form spores rather than stalk in chimeras with wild type

Using a selection for Dictyostelium mutants that preferentially form spores, we have recovered a mutant called CheaterA. In chimeras with isogenic wild-type cells, the CheaterA mutant preferentially forms viable spores rather than inviable stalk cells. The mutant causes wild-type cells that have begun to express spore-specific genes to accumulate in the prestalk compartment of the developing organism. In the wild-type cells, the chtA transcript is absent in growing cells and appears early in development. No transcript was detected in the mutant by Northern blot. The chtA gene codes for a protein with an F-box and WD40 domains. This class of protein usually forms part of an Skp1, cullin, F-box (SCF) complex that targets specific protein substrates for ubiquitination and degradation.

Dictyostelium amoebae lacking an F-box protein form spores rather than stalk in chimeras with wild type

Using a selection for Dictyostelium mutants that preferentially form spores, we have recovered a mutant called CheaterA. In chimeras with isogenic wild-type cells, the CheaterA mutant preferentially forms viable spores rather than inviable stalk cells. The mutant causes wild-type cells that have begun to express spore-specific genes to accumulate in the prestalk compartment of the developing organism. In the wild-type cells, the chtA transcript is absent in growing cells and appears early in development. No transcript was detected in the mutant by Northern blot. The chtA gene codes for a protein with an F-box and WD40 domains. This class of protein usually forms part of an Skp1, cullin, F-box (SCF) complex that targets specific protein substrates for ubiquitination and degradation.

An adenylyl cyclase that functions during late development of Dictyostelium

A variety of extracellular signals lead to the accumulation of cAMP which can act as a second message within cells by activating protein kinase A (PKA). Expression of many of the essential developmental genes in Dictyostelium discoideum are known to depend on PKA activity. Cells in which the receptor-coupled adenylyl cyclase gene, acaA, is genetically inactivated grow well but are unable to develop. Surprisingly, acaA(−) mutant cells can be rescued by developing them in mixtures with wild-type cells, suggesting that another adenylyl cyclase is present in developing cells that can provide the internal cAMP necessary to activate PKA. However, the only other known adenylyl cyclase gene in Dictyostelium, acgA, is only expressed during germination of spores and plays no role in the formation of fruiting bodies. By screening morphological mutants generated by Restriction Enzyme Mediated Integration (REMI) we discovered a novel adenylyl cyclase gene, acrA, that is expressed at low levels in growing cells and at more than 25-fold higher levels during development. Growth and development up to the slug stage are unaffected in acrA(−) mutant strains but the cells make almost no viable spores and produce unnaturally long stalks. Adenylyl cyclase activity increases during aggregation, plateaus during the slug stage and then increases considerably during terminal differentiation. The increase in activity following aggregation fails to occur in acrA(−) cells. As long as ACA is fully active, ACR is not required until culmination but then plays a critical role in sporulation and construction of the stalk.

Signalling pathways that direct prestalk and stalk cell differentiation in Dictyostelium

Prestalk cell differentiation in Dictyostelium is induced by DIF and two DIF-induced genes, ecmA and ecmB, have revealed the existence of multiple prestalk and stalk cell sub-types. These different sub-types are defined by the pattern of expression of subfragments derived from the ecmA and ecmB promoters. These markers have been utilised in three ways; for fate mapping in vivo, to investigate the molecular mechanisms underlying DIF signalling and to explore the relative requirement for DIF and other signalling molecules for prestalk and stalk cell differentiation in vitro. The heterogeneity of the prestalk and stalk populations seems to be reflected in differences in the cell signalling pathways that they utilise.

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.

A novel component involved in ubiquitination is required for development of Dictyostelium discoideum

A novel component of the ubiquitination system, called NOSA, is essential for cellular differentiation in Dictyostelium discoideum. Disruption of nosA does not affect the growth rate but causes an arrest in development after the cells have aggregated. nosA contains seven exons and codes for a developmentally regulated 3.5-kb mRNA. The 125-kDa NOSA protein is present in the cytosol at constant levels during growth and development. The C-terminal region of NOSA has homology with ubiquitin fusion degradation protein-2 (UFD2) of Saccharomyces cerevisiae and putative homologs in Caenorhabditis elegans and humans. UFD2 is involved in the ubiquitin-mediated degradation of model substrates in which ubiquitin forms part of the translation product, but ufd2 mutants have no detected phenotype. In accord with the homology to UFD2, we found differences in the ubiquitination patterns between nosA mutants and their parental cell line. While general in vivo and in vitro ubiquitination is minimally affected, ubiquitination of individual proteins is altered throughout growth and development in nosA mutants. These findings suggest that events involving ubiquitination are critical for progression through the aggregate stage of the Dictyostelium life cycle.

Calcium levels correlate with cell cycle phase and affect the level of the cyclin B transcript in Dictyostelium discoideum

In pre-aggregation amoebae of Dictyostelium discoideum, phenotypic differences with respect to cellular Ca2+ and cell cycle phases are known to bias post-aggregative cell-type choice. Using chlortetracycline fluorescence as an indicator, we found that cellular Ca2+ is highest at the S phase of the cell cycle. Upon increasing the level of Ca2+ with the help of the calcium ionophore A23187, there is a significant decrease in the cyclin B (clb1) mRNA level; the cdc2 mRNA level shows a marginal decrease. These results suggest that the effect of Ca2+ and the cell cycle on cell fate could be exerted at the level of transcription, or message stability, of specific genes.

Clathrin heavy chain is required for spore cell but not stalk cell differentiation in Dictyostelium discoideum

Previous studies of a clathrin-minus Dictyostelium cell line revealed important roles for clathrin heavy chain (clathrin) in endocytosis, secretion of lysosomal hydrolases and osmoregulation. In this paper, we examine the contribution of clathrin-mediated membrane traffic to development in Dictyostelium discoideum. Clathrin-minus cells were delayed in early development. When exposed to starvation conditions, clathrin-minus cells streamed and aggregated more slowly than wild-type cells. Although clathrin-minus cells displayed only 40% the level of extracellular cyclic AMP binding normally found in wild-type cells, they responded chemotactically to extracellular cyclic AMP. Clathrin-minus cells down-regulated cyclic AMP receptors, but only to half the extent of wild-type cells. We found that the extent of development of clathrin-minus cells was variable and influenced by environmental conditions. Although the mutant cells always progressed beyond the tipped mound stage, the final structure varied from a finger-like projection to a short, irregular fruiting body. Microscopic examination of these terminal structures revealed the presence of intact stalks but a complete absence of spores. Clathrin-minus cells expressed prestalk (ecmA and ecmB) and prespore (psA and cotB) genes normally, but were blocked in expression of the sporulation gene spiA. Using clathrin-minus cells that had been transformed with various promoter-lacZ reporter constructs, we saw only partial sorting of clathrin-minus prestalk and prespore cells. Even when mixed with wild-type cells, clathrin-minus cells failed to sort correctly and never constructed functional spores. These results suggest three roles for clathrin during Dictyostelium development. First, clathrin increases the efficiency of early development. Second, clathrin enables proper and efficient patterning of prestalk and prespore cells during culmination. Third, clathrin is essential for differentiation of mature spore cells.

Evidence that a combined activator-repressor protein regulates Dictyostelium stalk cell differentiation

The ecmA gene is expressed in Dictyostelium prestalk cells and is inducible by differentiation-inducing factor (DIF), a low-molecular-weight lipophilic substance. The ecmB gene is expressed in stalk cells and is under negative control by two repressor elements. Each repressor element contains two copies of the sequence TTGA in an inverted relative orientation. There are activator elements in the ecmA promoter that also contain two TTGA sequences, but in the same relative orientation. Gel retardation assays suggest that the same protein binds to the ecmB repressor and the ecmA activator. We propose that DIF induces prestalk cell differentiation by activating this protein and that the protein also binds to the promoters of stalk-specific genes, acting as a repressor that holds cells in the prestalk state until culmination is triggered.

Cloning, structural organization and regulation of expression of the Penicillium chrysogenum paf gene encoding an abundantly secreted protein with antifungal activity

An abundantly secreted, highly basic 12-kDa protein (PAF) was purified from the culture medium of Penicillium chrysogenum (Pc). Based on the N-terminal amino acid (aa) sequence of the protein, an oligodeoxyribonucleotide probe was derived and used for amplification of the encoding cDNA by PCR. This cDNA fragment encodes a Cys-rich preproprotein of 92 aa which appears to be processed to a mature product of 55 aa. The deduced aa sequence of the preproprotein reveals 42.6% identity to an antifungal protein (AFP) of Aspergillus giganteus. Agar diffusion tests confirmed that the Pc protein exhibits antifungal activity. In order to investigate the promoter region and the structural organization of the paf gene, a genomic 6-kb fragment was isolated and partially sequenced. Comparison of the nucleotide sequence of the genomic fragment and the cDNA clone revealed the presence of a coding region of 279 bp which is interrupted by two introns of 76 and 68 bp in length. In the promoter region, a typical TATA box, a motif resembling the fungal carbon catabolite repression element, as well as several putative GATA factor binding motifs, were found. Northern blot analysis indicated that the regulation of paf expression occurs at the level of mRNA transcription and is under control of carbon catabolite and nitrogen metabolite repression regulatory circuits.

The extracellular matrix of the Dictyostelium discoideum slug

In this review, we detail the current understanding of the extracellular matrix (ECM) of the migratory slug phase of the cellular slime mould, Dictyostelium discoideum. We describe some structural and non-structural molecules which comprise the ECM, and how these molecules reflect both plant and animal ECM systems. We also describe zones of the multicellular slug that are known to make ECM components, including the role of the prestalk cells and the slug epithelium-like layer. Finally, we review the contributions of studies on mutants to our understanding of the ECM of D. discoideum, and relate this to differentiation and development in more complex eukaryotic systems.

A protein kinase from Dictyostelium discoideum with an unusual acidic repeat domain

DdKinX codes for 1093 amino acids which are organized in four regions: the N-terminal catalytic domain, a region containing 30% acidic amino acids, tandem repeats of the motif VKVEEPVEE and the C-terminus. Identity with other protein kinases is 25 to 30%. Descendent trees show that DdKinX does not belong to any of the known kinase branches.

Metabolic pathways for differentiation-inducing factor-1 and their regulation are conserved between closely related Dictyostelium species, but not between distant members of the family

There is suggestive evidence that a conserved signalling system involving differentiation inducing factor-1 (DIF-1) controls stalk cell differentiation in a variety of slime mould species. In the standard laboratory species, Dictyostelium discoideum, DIF-1 is first inactivated by dechlorination catalysed by DIF-1 dechlorinase, then by several hydroxylation events, so that eventually about 12 metabolites are produced. If DIF-1 is used as a signal molecule in other species, they too must be able to metabolize it. We report here that the essentials of DIF-1 metabolism are conserved in D. mucoroides, the closest relative of D. discoideum. Both the dechlorinase and hydroxylase enzymes were present in D. mucoroides, and living cells of both species produced a similar spectrum of metabolites from [3H]DIF-1. Furthermore, DIF-1 dechlorinase was induced by DIF-1, as in D. discoideum, and this induction was repressed by ammonia and cAMP. DIF-1 dechlorinase could not be detected in cell extracts from D. minutum or Polysphondylium violaceum. However, living cells of both species are able to metabolize DIF-1; P. violaceum seems to produce a small amount of the monodechlorinated compound, DIF-3, but all other metabolites from both species appear to be unique. Thus all investigated species can metabolize DIF-1, but the exact route of metabolism is not highly conserved.

Universal signals control slime mold stalk formation

The primitive slime mold Dictyostelium minutum does not display oscillations during aggregation, cannot form migrating slugs, and does not form a prestalk/prespore pattern, all of which are characteristic for development of its advanced relative Dictyostelium discoideum. We used D. minutum to investigate whether slime molds share common mechanisms controlling development. In D. discoideum, the morphogen differentiation inducing factor (DIF) can induce stalk-cell differentiation in vitro. However, stalk formation in vivo is supposedly triggered by local depletion of DIF antagonists such as ammonia or cAMP. A homologue of the D. discoideum stalk gene ecmB was cloned in D. minutum that encodes a 3.4-kb mRNA, and its deduced amino acid sequence shows repeats of 24 amino acids that are characteristic for the D. discoideum ecmB gene. Remarkably, DIF effectively induces expression of the D. minutum ecmB gene and ammonia inhibits its expression. D. discoideum cells were transformed with a construct of the D. minutum ecmB promoter fused to the lacZ reporter gene and showed expression in the stalk, but not in the upper and lower cup of the fruiting body, which also express the D. discoideum ecmB gene. In D. discoideum, the D. minutum ecmB and the ecmB promoter are similarly activated by DIF and repressed by both cAMP and ammonia, suggesting that additional signaling is required for ecmB expression in upper and lower cup cells. Our data indicate that the extracellular signals controlling stalk formation and their intracellular signaling cascades including gene regulatory proteins remained highly conserved during slime mold evolution.

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.

Regulation of Dictyostelium morphogenesis by cAMP-dependent protein kinase

During formation of the Dictyostelium slug extracellular cAMP signals direct the differentiation of prespore cells and DIF, a chlorinated hexaphenone, induces the differentiation of prestalk cells. At culmination the slug transforms into a fruiting body, composed of a stalk supporting a ball of spores. A dominant inhibitor of cAMP-dependent protein kinase (PKA) expressed under the control of a prestalk-specific promoter blocks the differentiation of prestalk cells into stalk cells. Analysis of a gene specifically expressed in stalk cells suggests that PKA acts to remove a repressor that prevents the premature induction of stalk cell differentiation by DIF during slug migration. PKA is also necessary for the morphogenetic movement of prestalk cells at culmination. Expression of the PKA inhibitor under control of a prespore-specific promoter blocks the accumulation of prespore mRNA sequences and prevents terminal spore cell differentiation. Thus PKA is essential for progression along both pathways of terminal differentiation but with different mechanisms of action. On the stalk cell pathway it acts to regulate the action of DIF while on the spore cell pathway PKA itself seems to act as the inducer of spore cell maturation. Ammonia, the extracellular signal which regulates the entry into culmination, acts by controlling the intracellular concentration of cAMP and thus exerts its effects via PKA. The fact that PKA is necessary for both prespore and spore gene expression leads us to postulate the existence of a signalling mechanism which converts the progressive rise in cAMP concentration during development into discrete, PKA-regulated gene activation events.

Transcriptional regulation by folate: inducible gene expression in Dictyostelium transformants during growth and early development

The Dictyostelium discoidin genes are induced in bacteria-grown cells shortly before the onset of development but are also highly expressed during growth in axenic medium. We here show that axenically growing cells strongly respond to the extracellular signal folate by suppressing discoidin synthesis while cell growth and development is not substantially affected. Repression occurs via two previously identified promoter elements, the dIE and the dAXE. Removal of the signal molecules or setting cells up for development results in rapid reactivation of the promoter. Based on this observation, we constructed the transformation vector pVEII and describe a convenient method which allows for controlled expression of a gene of interest in growing cells and also for external modulation in early development. Deletion constructs of the discoidin promoter can be used in addition to vary transcriptional activity over about one order of magnitude.

A G-rich sequence element common to Dictyostelium genes which differ radically in their patterns of expression

Removal of a G-rich element from the DIF-inducible, prestalk-, and stalk-specific ecmB gene reduces expression but cell-type specificity is retained. The ecmB element will functionally substitute for a homologous sequence upstream of CP2, a cAMP-inducible gene and is bound by GBF, the factor which interacts with the CP2 G box. These results suggest that the G box may play a similar stimulatory role in these two independently regulated genes where it presumably interacts with different ancillary promoter elements.

Cyclic nucleotide phosphodiesterase of Dictyostelium discoideum and its glycoprotein inhibitor: structure and expression of their genes

The genes coding for the cyclic nucleotide phosphodiesterase (PD) and the PD inhibitory glycoprotein (PDI) have been cloned and characterized. The PDI gene was isolated as a 1.6 kb genomic fragment, which included the coding sequence containing two small introns and 510 nucleotides of non-translated 5' sequence. From the deduced amino acid sequence we predict a protein with a molecular weight (MW) of 26,000 that, in agreement with previous data, contains 15% cysteine residues. Genomic Southern blot analysis indicates that only one gene encodes the inhibitor. Northern blot analysis shows a single transcript of 0.95 kb. The PDI gene is expressed early in development with little transcript remaining following aggregation. The appearance of PDI mRNA is prevented by the presence of cAMP, but when cAMP is removed the transcript appears within 30 minutes. When cAMP is applied to cells expressing PDI the transcript disappears with a half-life of less than 30 minutes. The PD gene of D. discoideum is transcribed into three mRNAs: a 1.9 kb mRNA specific for growth, a 2.4 kb mRNA specific for aggregation, and a 2.2 kb mRNA specific for late development. The 2.2 kb mRNA is also specific for prestalk cells, and is induced by differentiation-inducing factor. All three mRNAs contain the same coding sequence, and differ only in their 5' non-coding sequences. Each mRNA is transcribed from a different promoter, and by using the chloramphenicol acyltransferase gene as a reporter, we have shown that each promoter displays the same regulation as its cognate mRNA.(ABSTRACT TRUNCATED AT 250 WORDS)

A developmentally regulated and cAMP-repressible gene of Dictyostelium discoideum: cloning and expression of the gene encoding cyclic nucleotide phosphodiesterase inhibitor

A 1.6-kb genomic fragment containing the coding region for the inhibitor (PDI) of cyclic nucleotide phosphodiesterase (PD) was isolated and sequenced. The genomic sequence includes 510 nucleotides (nt) of 5'-noncoding sequence and the full coding sequence, which contains two small introns. From the deduced amino acid (aa) sequence we predict a 26-kDa protein that, in agreement with previous data, contains approximately 15% Cys residues. The PDI possesses a hydrophobic leader sequence, five potential glycosylation sites, and three internal repeats. Northern-blot analysis showed a single transcript of 0.95 kb. The gene encoding PDI (pdi) was expressed early in development with little transcript remaining following aggregation. The appearance of pdi transcript was inhibited by cAMP, but when cAMP was removed the transcript appeared within 30 min. When cAMP was applied to cells containing pdi mRNA, the transcript disappeared with a half-life of less than 30 min.

Cytoplasmic acidification facilitates but does not mediate DIF-induced prestalk gene expression in Dictyostelium discoideum

Stalk cell differentiation in Dictyostelium can be induced by the differentiation-inducing factor, DIF, or by conditions that decrease intracellular pH (pHi). We have investigated whether cytoplasmic acidification acts directly to induce expression of pDd56 and pDd63, two DIF-regulated genes, specifically expressed in prestalk cells. The weak base methylamine, which increases pHi, inhibits DIF-induced transcription. The weak acid 5,5-dimethyl-2,4-oxazolidinedione (DMO), which decreases pHi, stimulates DIF-induction of the two prestalk genes. After relatively long incubation periods, DMO also induces a low level of prestalk gene expression in the absence of added DIF. However, unlike DIF-mediated induction, the apparent DMO-mediated induction decreases to undetectable levels when the cell density is reduced from 10(7) to 10(5) cells/ml. This indicates that DMO does not itself induce gene expression, but acts to enhance the effects of an autonomously secreted stalk-inducing factor, presumably DIF. These results suggest that the effects of DIF on gene expression are regulated by intracellular pH, but do not support a role for protons as direct intermediates in the DIF signal transduction pathway.

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.

Expression and organization of BP74, a cyclic AMP-regulated gene expressed during Dictyostelium discoideum development

We have characterized a cDNA and the corresponding gene for a cyclic AMP-inducible gene expressed during Dictyostelium development. This gene, BP74, was found to be first expressed about the time of aggregate formation, approximately 6 h after starvation. Accumulation of BP74 mRNA did not occur in Dictyostelium cells that had been starved in fast-shaken suspension cultures but was induced in similar cultures to which cyclic AMP pulses had been added. The BP74 cDNA and gene were characterized by DNA sequence analysis and transcriptional mapping. When the BP74 promoter region was fused with a chloramphenicol acetyltransferase reporter gene and reintroduced into Dictyostelium cells, the transfected chloramphenicol acetyltransferase gene displayed the same developmentally regulated pattern of expression as did the endogenous BP74 gene, suggesting that all of the cis-acting elements required for regulated expression were carried by a 2-kilobase cloned genomic fragment. On the basis of sequence analysis, the gene appeared to encode a protein containing a 20-residue hydrophobic sequence at the amino-terminal end and 26 copies of a 20-amino-acid repeat.

The cyclic nucleotide phosphodiesterase gene of Dictyostelium discoideum utilizes alternate promoters and splicing for the synthesis of multiple mRNAs

The cyclic nucleotide phosphodiesterase (phosphodiesterase) gene plays essential roles in the development of Dictyostelium discoideum during cellular aggregation and postaggregation morphogenesis. Genomic clones spanning the gene were isolated and used to determine the sequence and structure of the phosphodiesterase gene. We found an unusually complex organization for a gene of D. discoideum. Two transcripts of 2.4 and 1.9 kilobases (kb) were synthesized from start sites separated by 1.1 kb. A developmentally regulated promoter was utilized for the 2.4-kb mRNA, and a constitutive promoter regulated synthesis of the 1.9-kb transcript. The gene was found to be divided into four exons that are alternately spliced to give rise to the two mRNAs. The precursor of the 2.4-kb mRNA contained a 2.3-kb intron, whereas the precursor of the constitutive transcript was synthesized with a 1.7-kb intron. The two transcripts contained identical protein-coding regions and 400-nucleotide 3' untranslated sequences. The 2.4-kb developmentally regulated mRNA was distinguished by a long 5' untranslated leader of 666 nucleotides. The complex structure of the gene may allow multiple levels of control of the expression of the phosphodiesterase during development.

Expression and organization of BP74, a cyclic AMP-regulated gene expressed during Dictyostelium discoideum development

We have characterized a cDNA and the corresponding gene for a cyclic AMP-inducible gene expressed during Dictyostelium development. This gene, BP74, was found to be first expressed about the time of aggregate formation, approximately 6 h after starvation. Accumulation of BP74 mRNA did not occur in Dictyostelium cells that had been starved in fast-shaken suspension cultures but was induced in similar cultures to which cyclic AMP pulses had been added. The BP74 cDNA and gene were characterized by DNA sequence analysis and transcriptional mapping. When the BP74 promoter region was fused with a chloramphenicol acetyltransferase reporter gene and reintroduced into Dictyostelium cells, the transfected chloramphenicol acetyltransferase gene displayed the same developmentally regulated pattern of expression as did the endogenous BP74 gene, suggesting that all of the cis-acting elements required for regulated expression were carried by a 2-kilobase cloned genomic fragment. On the basis of sequence analysis, the gene appeared to encode a protein containing a 20-residue hydrophobic sequence at the amino-terminal end and 26 copies of a 20-amino-acid repeat.

The cyclic nucleotide phosphodiesterase gene of Dictyostelium discoideum utilizes alternate promoters and splicing for the synthesis of multiple mRNAs

The cyclic nucleotide phosphodiesterase (phosphodiesterase) gene plays essential roles in the development of Dictyostelium discoideum during cellular aggregation and postaggregation morphogenesis. Genomic clones spanning the gene were isolated and used to determine the sequence and structure of the phosphodiesterase gene. We found an unusually complex organization for a gene of D. discoideum. Two transcripts of 2.4 and 1.9 kilobases (kb) were synthesized from start sites separated by 1.1 kb. A developmentally regulated promoter was utilized for the 2.4-kb mRNA, and a constitutive promoter regulated synthesis of the 1.9-kb transcript. The gene was found to be divided into four exons that are alternately spliced to give rise to the two mRNAs. The precursor of the 2.4-kb mRNA contained a 2.3-kb intron, whereas the precursor of the constitutive transcript was synthesized with a 1.7-kb intron. The two transcripts contained identical protein-coding regions and 400-nucleotide 3' untranslated sequences. The 2.4-kb developmentally regulated mRNA was distinguished by a long 5' untranslated leader of 666 nucleotides. The complex structure of the gene may allow multiple levels of control of the expression of the phosphodiesterase during development.

A new anatomy of the prestalk zone in Dictyostelium

The characteristic structure of the mature Dictyostelium culminant is created by the regionalized cellular differentiation and directed movement of prestalk cells. The front prestalk zone of the migratory slug has previously been considered to be a homogeneous tissue. Here we demonstrate, however, the existence of multiple classes of prestalk cells located in different parts or the slug anterior. The pDd56 and pDd63 genes encoding closely related extracellular matrix proteins are dependent for their expression upon DIF-1, the specific stalk-cell inducer. We have fused the promoters of the two genes to a modified chloramphenicol acetyltransferase (cat) gene to produce immunologically detectable proteins which localize to the cell nucleus. These two markers define three distinct kinds of 'prestalk' cells. One class, which we term 'prestalk A' cells, expressed the pDd63 gene. 'Prestalk B' cells express pDd56 and may also express the pDd63 gene. A third class, which we term 'prestalk 0' cells, expresses neither marker.

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.

Identification and localization of proteins encoded by two DIF-inducible genes of Dictyostelium

We show that pDd56 and pDd63, two related DIF-inducible genes of Dictyostelium, respectively encode the ST310 and ST430 polypeptides identified by Morrissey, Devine, and Loomis (1984, Dev. Biol. 103, 414-424). We localize the two proteins by immunoelectron microscopy to the extracellular matrix surrounding the stalk cells and the stalk tube. Coupled with their predicted amino acid sequence and biochemical properties, this suggests that they are structural proteins of the stalk.