Growing and developing Dictyostelium cells express different ras genes

SAS1 and SAS2, GTP-binding protein genes in Dictyostelium discoideum with sequence similarities to essential genes in Saccharomyces cerevisiae

We have identified two novel, very closely related genes, SAS1 and SAS2, from Dictyostelium discoideum. These encode small, approximately 20-kilodaton proteins with amino acid sequences thought to be involved in interaction with guanine nucleotides. The protein sizes, spacings of GTP-binding domains, and carboxyl-terminal sequences suggest their relationship to the ubiquitous ras-type proteins. Their sequences, however, are sufficiently different to indicate that they are not true ras proteins. More extensive sequence identity (approximately 55%) is shared with the YPT1 and SEC4 proteins from Saccharomyces cerevisiae. These yeast proteins are essential for growth and are believed to be involved in intracellular signaling associated with membrane function. SAS1 and SAS2 exhibit distinct patterns of genomic organization and developmentally regulated gene expression. SAS1 contains introns and is associated with a developmentally regulated repetitive element. SAS2 is colinear with its mRNA and does not appear to be closely linked with this repetitive element. Both genes are expressed during growth and throughout development. SAS1 is maximally expressed during cytodifferentiation, when two sizes of SAS1 mRNA are detectable. SAS2 mRNA levels are maximal during culmination. On the basis of the expression patterns of the SAS genes and their relationship to the YPT1 and SEC4 genes, we discuss possible functions of the SAS proteins.

SAS1 and SAS2, GTP-binding protein genes in Dictyostelium discoideum with sequence similarities to essential genes in Saccharomyces cerevisiae

We have identified two novel, very closely related genes, SAS1 and SAS2, from Dictyostelium discoideum. These encode small, approximately 20-kilodaton proteins with amino acid sequences thought to be involved in interaction with guanine nucleotides. The protein sizes, spacings of GTP-binding domains, and carboxyl-terminal sequences suggest their relationship to the ubiquitous ras-type proteins. Their sequences, however, are sufficiently different to indicate that they are not true ras proteins. More extensive sequence identity (approximately 55%) is shared with the YPT1 and SEC4 proteins from Saccharomyces cerevisiae. These yeast proteins are essential for growth and are believed to be involved in intracellular signaling associated with membrane function. SAS1 and SAS2 exhibit distinct patterns of genomic organization and developmentally regulated gene expression. SAS1 contains introns and is associated with a developmentally regulated repetitive element. SAS2 is colinear with its mRNA and does not appear to be closely linked with this repetitive element. Both genes are expressed during growth and throughout development. SAS1 is maximally expressed during cytodifferentiation, when two sizes of SAS1 mRNA are detectable. SAS2 mRNA levels are maximal during culmination. On the basis of the expression patterns of the SAS genes and their relationship to the YPT1 and SEC4 genes, we discuss possible functions of the SAS proteins.

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

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

Regulation of DdrasG gene expression during Dictyostelium development

DdrasG gene expression during the early development of Dictyostelium discoideum has been examined in detail. The amount of DdrasG-specific mRNA increased approximately twofold during the first 2 to 3 h of development and then declined rapidly, reaching negligible levels by the aggregation stage. The increase in mRNA levels that occurred during the first 2 to 3 h of development also occurred during differentiation in cell suspensions and was enhanced when cells were shaken rapidly. This initial increase was unaffected by cell density. When cells were set up to differentiate on filters, the addition of a glucose-amino acid mixture slightly delayed differentiation and had a similar effect on the expression of the gene. The decline in DdrasG expression during development did not occur when cells were treated with cycloheximide, suggesting that the expression of a developmentally regulated gene product is essential for the reduction of DdrasG gene mRNA. There was no decrease in DdrasG mRNA level during differentiation in shake suspension, but the decrease did occur upon application of pulses of cyclic AMP to shaking cultures. The application of a continuously high level of cyclic AMP delayed the increase in expression of the gene and did not result in the subsequent decline. These results suggest that the induction of a functional cyclic AMP relay system is important in reducing DdrasG gene mRNA levels.

Regulation of DdrasG gene expression during Dictyostelium development

DdrasG gene expression during the early development of Dictyostelium discoideum has been examined in detail. The amount of DdrasG-specific mRNA increased approximately twofold during the first 2 to 3 h of development and then declined rapidly, reaching negligible levels by the aggregation stage. The increase in mRNA levels that occurred during the first 2 to 3 h of development also occurred during differentiation in cell suspensions and was enhanced when cells were shaken rapidly. This initial increase was unaffected by cell density. When cells were set up to differentiate on filters, the addition of a glucose-amino acid mixture slightly delayed differentiation and had a similar effect on the expression of the gene. The decline in DdrasG expression during development did not occur when cells were treated with cycloheximide, suggesting that the expression of a developmentally regulated gene product is essential for the reduction of DdrasG gene mRNA. There was no decrease in DdrasG mRNA level during differentiation in shake suspension, but the decrease did occur upon application of pulses of cyclic AMP to shaking cultures. The application of a continuously high level of cyclic AMP delayed the increase in expression of the gene and did not result in the subsequent decline. These results suggest that the induction of a functional cyclic AMP relay system is important in reducing DdrasG gene mRNA levels.