B-MYB FUNCTION CAN BE MARKEDLY ENHANCED BY CYCLIN A-DEPENDENT KINASE AND PROTEIN TRUNCATION

Transcription of the B-Myb gene is cell cycle regulated by an E2F-depe ndent mechanism and its product, B-Myb, is itself a transcription fact or required for S-phase entry. Previously, rye have shown that B-Myb i s specifically phosphorylated during S-phase and that similar modifica tion to a less electrophoretically mobile form could be induced by bac ulovirus-expressed cyclin A/Cdk2 kinase. We report here that cyclin A- mediated phosphorylation of B-Myb is associated with a marked increase in transactivation function in U-2 OS cells. In contrast to previous studies, transactivation of the luciferase reporter was dependent upon Myb binding sites located upstream of the promoter. Enhancement of B- Myb activation function was also obtained by truncation of the C-termi nus just downstream of a domain conserved in evolution. Potentiation o f B-Myb activity by phosphorylation was not simply a consequence of ov ercoming the negative effect of the C-terminus, however, as the trunca ted protein was to a lesser extent also activated by cyclin A/Cdk2. Wh ereas wild-type B-Myb transactivation activity could not be potentiate d by cyclin A/Cdk2 in NIH3T3 cells, the truncated protein was hyperact ive. Finally, we showed that B-Myb synergises with cyclin A to promote U-2 OS cells into S-phase.