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 mutant of Rous sarcoma virus in quiescent Rat-1 cells leads to passage through the cell cycle. This is accompanied by a transient increase of the DNA binding activity of the transcription factor AP-1 which is not sufficient for the v-Src mediated cell cycle traverse. There is another need for v-Src later in the G1 phase of the cycle, and after completion of that event, cells are able to progress through DNA synthesis and division in the absence of either v-Src or other growth factors. When 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 factors.