CHARACTERIZATION OF THE K-RAS GENE OF THE MARSUPIAL MONODELPHIS-DOMESTICA

A cDNA clone isolated from a plasmid library contains the complete cod ing sequence for the K-ras gene of the marsupial Monodelphis domestica , a South American opossum. The nucleotide sequence of the coding regi on of the opossum K-ras gene is very similar to the K-ras coding seque nces of placental mammals. The coding region of the opossum gene is 95 % identical to the human gene at the nucleotide level; the human and o possum genes are 99% identical at the level of encoded amino acids. Tr anscribed but untranslated regions of the opossum gene 3' and 5' to th e coding region are similar to corresponding regions of the human and mouse genes, but are less highly conserved than translated sequences. Based on the nucleotide sequence of the opossum K-ras cDNA clone, prim ers were designed that allowed amplification of exons 1 and 2 of the g ene from opossum genomic DNA by the polymerase chain reaction. When ex ons 1 and 2 of K-ras were amplified from DNA isolated from an ultravio let radiation-induced eye tumor of M. domestica and the nucleotide seq uence of amplified material was determined, a heterozygous mutation in codon 61 of the gene was detected. This T to A transversion resulted in a change in the amino acid encoded by the codon. The tumor from whi ch DNA was isolated had previously been shown to contain a transformin g K-ras oncogene. Thus, the opossum K-ras gene can be mutationally act ivated in a manner similar to the K-ras genes of placental mammals.