Expression of c-myc with constitutively active mutants of the ras gene
results in the cooperative transformation of primary fibroblasts, alt
hough the precise mechanism by which these genes cooperate is unknown.
Since c-Myc has been shown to Function as a transcriptional activator
, we have examined the ability of c-Myc e and activated Ras (H-Ras(V-1
2)) to cooperatively induce the promoter activity of cdc2, a gene whic
h is critical for cell cycle progression. Microinjection of expression
constructs encoding H-Ras(V-12) and c-Myc along with a cdc2 promoter-
luciferase reporter plasmid into quiescent cells led to an increase in
cdc2 promoter activity approximately 30 h after injection, a period w
hich coincides with the S-to-G(2)/M transition in these cells. Express
ion of H-Ras(V-12) alone weakly activated the cdc2 promoter, while exp
ression of c-Myc alone had no effect. Mutants of c-Myc lacking either
the leucine zipper dimerization domain or the phosphoacceptor site Ser
-62 could not cooperate with H-Ras(V-12) to induce the cdc2 promoter.
These mutants also lacked the ability to cooperate with H-Ras(V-12) to
stimulate DNA synthesis. Deletion analysis identified a distinct regi
on of the cdc2 promoter which was required for c-Myc responsiveness. T
aken together, these observations suggest a mechanistic link between t
he molecular activities of c-Myc and Ras and induction of the cell cyc
le regulator Cdc2.