Viral oncogenes and cellular prototypes.

The structural hallmark of retroviral transforming onc genes is a specific RNA sequence that is unrelated to the essential retroviral genes but closely related to certain cellular prototypes termed proto-onc genes. Two types of onc genes have been distinguished. Type I are onc genes which utilize elements of specific sequences only to encode a transforming protein. Type I1 onc genes are hybrids which utilize essential viral (typically gag) and specific RNA sequences to encode transforming proteins. Comparisons between viral onc genes and cellular proto-onc genes are reviewed in the light of two competing models for protoonc function: the quantitative model, which holds that viral onc genes and cellular proto-onc genes are functionally the Same and that transformation is the result of enhanced dosage of a cellular proto-onc gene; and the qualitative model, which holds that they are different. Structural comparisons between viral onc genes and cellular prototypes have demonstrated extensive sequence homologies in the primary structures of the specific sequences. However, qualitative differences exist in the structure and organization of viral onc genes and cellular prototypes. These include differences in promoters, minor differences in the primary structure of shared sequences, and absolute differences such as in the