Two ras genes isolated from the yeast strain Saccharomyces cerevisia have been designated c-rasI and c-rasII. Both genes are transcriptionally regulated. Carbon sources appear to play a major role in the regulation of both genes. The c-rasI transcript is induced by dextrose very early in a growth cycle, whereas the transcript is absent when cells reach mid log phase. There are three major c-rasII transcripts differing only in the location of their 5' ends. This gene appears to be regulated primarily by selection of a particulat 5' end. Yeast cells in which only the c-rasI gene is present have been shown to sporulate in rich media in the presence of dextrose. We believe this is due to the absence of c-ras gene product because there is essentially no c-rasI transcript present in late log phase. Thus, the cells are starved even in rich media. We have shown that cells lacking the c-rasII gene do not grow in synthetic media containing non-fermentable carbon sources (e.g. acetone, glycerol, ethanol). When cells contain a c-rasII Val-19 gene instead of the wild type c-rasII gene, growth in synthetic media occurs in the presences of dextrose, but cells begin to die when they reach late log phase. Cells do not grow in non-fermentable carbon sources, but instead die. We have also observed the absence of regulation of c-rasII transcripts in these cells.
