Abstract
The rat-derived Harvey murine sarcoma virus (Ha-MuSV) contains a transduced ras oncogene activated by two missense mutations and flanked by rat retroviruslike VL30 sequences. Ha-MuSV induces focal transformation of mouse NIH 3T3 cells in vitro and tumors (fibrosarcomas and splenic erythroleukemias) in newborn mice. We have used these two assays to study the contribution of coding and noncoding viral sequences to the biological activity of Ha-MuSV. A good correlation was found between the in vitro and in vivo assays. In several different isogenic Ha-MuSV variants, those with a rasH gene that had one or both of the Ha-MuSV missense mutations were much more active biologically than the corresponding proto-oncogene. A Ha-MuSV variant that encoded the proto-oncogene protein induced lymphoid leukemias (with thymomas), with a relatively long latent period, rather than the fibrosarcomas and erythroleukemias characteristic of Ha-MuSV with one or both missense mutations. A VL30-derived segment with enhancer activity was identified downstream from v-rasH. A mutant Ha-MuSV from which this 3' noncoding segment was deleted expressed lower levels of the wild-type viral protein, displayed impaired transforming activity in vitro, and induced lymphoid leukemias (with thymomas). 5' noncoding rat c-rasH sequences were found to increase the biological activity of the virus when substituted for the corresponding segment of v-rasH. We conclude that (i) the biological activity of Ha-MuSV can be influence significantly by noncoding sequences located outside the long terminal repeat as well as by coding sequences, (ii) VL30 sequences positively regulate the expression of v-rasH, (iii) relatively low biological levels of ras, whether resulting from low-level expression of wild type v-rasH or high-levels of ras proto-oncogene protein, induce a type of tumor that differs from tumors induced by high biological levels of ras, and (iv) the in vivo pathogenicity of the Ha-MuSV variants correlated with their transforming activity on NIH 3T3 cells.