Characterization of a third ras gene, rasB, that is expressed throughout the growth and development of Dictyostelium discoideum.

Previous reports have indicated that the cellular slime mold Dictyostelium discoideum possesses two ras genes (rasG and rasD) and one rap gene (rap1). All three genes are developmentally regulated, with each showing a different pattern of transcription during the Dictyostelium life cycle. To establish whether there are additional ras or rap genes in Dictyostelium, we used degenerate oligonucleotide primers to the highly conserved GTP-binding domains and both ras- and rap-unique sequences to amplify products from cDNA using the polymerase chain reaction (PCR). No additional rap genes were amplified, but a fragment whose nucleotide sequence predicted a novel ras gene was isolated. Using this PCR product as a probe, a full-length cDNA clone was isolated and sequenced. Its deduced amino acid sequence predicted a 197 amino acid protein that is 71% and 68% identical to RasG and RasD respectively. The new ras gene contains the conserved Ras-specific effector domain, the conserved binding site for the Ras-specific Y13-259 monoclonal antibody, and shows greater sequence similarity to the human H-Ras protein than to any other mammalian Ras protein. In view of this high level of identity to the ras gene subfamily, we have designated this gene rasB. Northern blot analysis has shown that rasB is developmentally regulated with maximum levels of a single 950-bp message detected during vegetative growth and the first 8 h of development.