Cyclin A1 activates B-myb to transactivate myb-site containing promoters.

A luciferase construct containing 3 myb-consensus binding sites upstream of a minimal thymidine kinase promoter was transiently transfected into Cos cells (left side). Whereas no significant activation of this construct was detected by either cyclin A1 or B-myb transfection alone, a more than 60-fold increase in luciferase activity occurred upon cotransfection of cyclin A1 and B-myb expression vectors. In the right side of the figure, the human cyclin A1 promoter (bp −190 to +145) was analyzed for transactivation by B-myb alone or by the combination of cyclin A1 and B-myb. Cotransfection of B-myb alone decreased cyclin A1 promoter activity by an average of 20%. Cyclin A1 alone did not have a significant effect on cyclin A1 promoter activity. However, when cyclin A1 and B-myb were transfected together, cyclin A1 promoter activity was induced 3-fold.