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

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.

Integrated maps of the chromosomes in Dictyostelium discoideum

Detailed maps of the six chromosomes that carry the genes of Dictyostelium discoideum were constructed by correlating physically mapped regions with parasexually determined linkage groups. Chromosomally assigned regions were ordered and positioned by the pattern of altered fragment sizes seen in a set of restriction enzyme mediated integration-restriction fragment length polymorphism (REMI-RFLP) strains each harboring an inserted plasmid that carries sites recognized by NotI, SstI, SmaI, BglI and ApaI. These restriction enzymes were used to digest high molecular weight DNA prepared from more than 100 REMI-RFLP strains and the resulting fragments were separated and sized by pulsed-field gels. More than 150 gene probes were hybridized to blots of these gels and used to map the insertion sites relative to flanking restriction sites. In this way, we have been able to restriction map the 35 mb genome as well as determine the map position of more than 150 genes to with approximately 40 kb resolution. These maps provide a framework for subsequent refinement.

The highly divergent alpha- and beta-tubulins from Dictyostelium discoideum are encoded by single genes

As a step in the characterization of the microtubule system of Dictyostelium discoideum, we have isolated and sequenced full-length cDNA clones that encode the Dictyostelium alpha- and beta-tubulins, as well as the Dictyostelium alpha-tubulin gene. Southern blot analysis suggests that Dictyostelium is unusual in that its genome contains single alpha- and beta-tubulin genes, rather than the multi-gene family common in most eukaryotic organisms. The complete alpha-tubulin cDNA contains 1558 nucleotides, with an open reading frame, that encode a protein of 457 amino acids. The complete beta-tubulin cDNA contains 1572 nucleotides and encodes a protein of 456 amino acids. Analysis of the deduced protein sequences indicates that while there is a significant degree of sequence similarity between the Dictyostelium tubulins and other known tubulins, the Dictyostelium alpha-tubulin displays the greatest sequence divergence yet described. Single alpha- and beta-tubulin transcripts are detected by northern blot analysis during all stages of Dictyostelium development. The highest levels of message accumulate late in germinating spores and vegetative amoebae. Despite changes in alpha- and beta-tubulin mRNA levels, protein levels remain constant throughout development. We have expressed the carboxy-terminal two-thirds of the alpha- and beta-tubulins as trpE fusions in Escherichia coli and used this protein to produce polyclonal antisera specific for the Dictyostelium alpha- and beta-tubulins. These antisera recognize one alpha- and two beta-tubulin spots on western blots of 2-D gels and, by indirect immunofluorescence, both recognize the interphase and mitotic microtubule arrays in vegetative amoebae.

The Dictyostelium essential light chain is required for myosin function

A Dictyostelium mutant (7-11) that expresses less than 0.5% of wild-type levels of the myosin essential light chain (EMLC) has been created by overexpression of antisense RNA. Cells from 7-11 contain wild-type levels of the myosin heavy chain (MHC) and regulatory light chain (RMLC). Myosin isolated from 7-11 cells consists of the MHC with the RMLC associated in reduced stoichiometry, and binds to purified actin in an ATP-sensitive fashion. Purified 7-11 myosin displays calcium-activated ATPase activity with a Vmax about 15%-25% of that of wild type, and a Km for ATP of 27 +/- 5 microM versus 83 +/- 30 microM for wild type. At actin concentrations as high as 17 microM, 7-11 myosin displays greatly reduced actin-activated ATPase activity. Phenotypically, 7-11 cells resemble MHC mutants, growing poorly in suspension and becoming large and multinucleate. When starved for multicellular development, 7-11 cells take several hours longer than wild-type cells to aggregate. Although multicellular aggregates eventually form, they fail to develop further. The cells are also unable to cap receptors in response to Con A treatment. Since cells expressing the EMLC are phenotypically similar to MHC null mutants, the EMLC appears necessary for myosin function, at least in part because it is required for normal actin-activated ATPase activity.

V4, a gene required for the transition from growth to development in Dictyostelium discoideum

The V4 gene of Dictyostelium discoideum is regulated in a nutrient-dependent manner and is deactivated immediately upon the onset of development. V4 is expressed only during growth, but its expression is not required for growth. We propose that the V4 gene product plays a role in the transition from growth to development. We have tested this hypothesis by antisense mutagenesis. Cells transformed with a V4 antisense construct contained no detectable endogenous V4 mRNA. These cells grew normally, but they failed to aggregate. Under conditions which normally promote development, V4 antisense transformants failed to deactivate vegetative-specific genes. These cells also were unable to induce the expression of the cAMP cell surface receptor, the cyclic nucleic phosphodiesterase, and contact sites A, all of which are normally induced under such conditions. Surprisingly, cells transformed with a V4 sense construct displayed a similar morphological and biochemical phenotype as the antisense cells, whereas cells transformed with the parental vector exhibited a normal biochemical and morphological phenotype. These results demonstrate that expression of the V4 gene during growth is required for the proper initiation of development.

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.

Dictyostelium discoideum essential myosin light chain: gene structure and characterization

We have used a Dictyostelium essential myosin light chain (EMLC) cDNA clone to isolate additional cDNA clones which supply a different 3' sequence from that previously described. The revised cDNA sequence encodes a polypeptide of 150 amino acids. Amino acid residues 147-167 of the previously reported sequence are replaced by new residues 147 to 150. The new cDNA encodes a polypeptide with 66% amino acid sequence identity with the Physarum polycephalum EMLC, and approximately 30% identity with mammalian EMLC sequences. These new cDNA clones were used to isolate two genomic DNA fragments which contain the entire EMLC gene. The Dictyostelium EMLC gene contains a single intron located immediately 3' of the translation initiation codon and encodes a product most similar to MLC3 isoform of vertebrates. Primer extension analysis places the transcription initiation site approximately 90 nucleotides upstream of the translation initiation site. A DNA fragment containing 350 bases of sequence upstream of the putative transcription initiation site is sufficient to drive expression of a reporter gene upon reintroduction into growing Dictyostelium cells. In addition, the CAT reporter mRNA produced by this construct showed a pattern of developmental regulation similar to that previously reported for the endogenous EMLC mRNA. Based on comparison with published EMLC sequences from a variety of sources, the Dictyostelium EMLC shows slightly higher similarity to vertebrate EMLCs from striated muscle sources than nonmuscle sources. While Dictyostelium and human nonmuscle sequences display only 28% identity over their entire sequence, the region from residue 88 to 108 shows much higher identity (67%). The high evolutionary conservation of this region of the EMLC suggests it may play an important role in EMLC function, and as such, represents a good target for future mutagenesis studies.

A pleiotropic defect in cAMP-regulated gene expression in the Dictyostelium agg- mutant synag 7

During differentiation of Dictyostelium discoideum, cAMP functions as a diffusible, extracellular signal to direct chemotaxis and regulate developmental gene expression. The availability of signal-transduction mutants of Dictyostelium now makes it feasible to pursue a genetic analysis of cAMP signaling. The synag 7 mutant is defective in receptor-mediated adenylate cyclase stimulation and cannot relay a cAMP signal. To further characterize this mutant, mRNA levels of several cAMP-regulated genes were measured during development. cAMP-regulated gene expression was found to be dramatically altered in synag 7:several different genes which require cAMP for expression in wild-type cells were induced in synag 7 in the absence of cAMP. In addition, the gene-encoding discoidin I, which is normally expressed in starved cells and repressed by cAMP, is expressed at very low levels in starved synag 7 cells, possibly due to precocious repression. These results suggest that a pleiotropic regulator of cAMP-regulated gene expression is uncoupled from its normal controls during development in synag 7.