MITOGENESIS BY V-SRC - A NEED FOR ACTIVE ONCOPROTEIN BOTH IN LEAVING G0 AND IN COMPLETING G1 PHASES OF THE CELL-CYCLE

Activation of the tyrosine kinase of a temperature sensitive v-Src mut ant of Rous sarcoma virus in quiescent Rat-I cells leads to passage th rough the cell cycle. This is accompanied by a transient increase of t he DNA binding activity of the transcription factor AP-1 which is not sufficient for the v-Src mediated cell cycle traverse. There is anothe r need for v-Src later in the G1 phase of the cycle, and after complet ion of that event, cells are able to progress through DNA synthesis an d division in the absence of either v-Src or other growth factors. Whe n cells are exposed to v-Src activity for periods insufficient for it to behave as a complete mitogen, it can act as either a competence or progression factor in conjunction with appropriate purified growth fac tors.