CLONING AND GENOMIC SEQUENCE OF THE PHYSARUM-POLYCEPHALUM PPRAS1 GENE, A HOMOLOG OF THE RAS PROTOONCOGENE

We have cloned the genomic copy of the Ppras1 gene, a homologue of the ras proto-oncogene, from the true slime mold Physarum polycephalum. P pras1 contains five small introns, four of which have a high content o f pyrimidines. The (dC)-homopolymers present in introns 4 and 5 may be responsible for the observed recA-independent deletion in Ppras1 upon amplification of the Ppras1-bearing plasmid by chloramphenicol. Altho ugh Ppras1 exhibits high amino acid and nucleotide homologies with the DdrasG gene, a homologue of ras from another slime mold, Dictyosteliu m discoideum, locations and sequences of their introns are quite diffe rent. This discordance suggests that introns of the ras genes in these species were acquired independently.