The promoters of genes which regulate entry into and progress through mitosis are typically induced maximally in G2 by transcription factors that include B-Myb and FoxM1. As FoxM1 gene transcription is a target of B-Myb, we investigated in this study how these transcription factors functionally interact to regulate these G2/M genes. Using a 3T3 cell line containing floxed B-myb alleles (B-myb(F/F)) that could be conditionally deleted by Cre recombinase, we confirmed that B-myb knockout caused both decreased mRNA expression of several G2/M genes, including FoxM1, and delayed entry into mitosis. Although FoxM1 protein expression was actually unaffected by B-myb knockout when quiescent B-myb(F/F) 3T3 cells re-entered the cell cycle upon serum-stimulation, chromatin immunoprecipitation revealed that FoxM1 binding to G2/M promoters was substantially reduced. FoxM1 transcriptional activity requires sequential phosphorylation by Cyclin-dependent kinases and Plk1, which are B-Myb target genes, and we found that phosphorylation at Plk1-specific sites was somewhat reduced upon B-myb knockout. Neither this effect nor nuclear accumulation of FoxM1, which was unaffected by B-myb knockout, was sufficient to account for the dependence on B-Myb for FoxM1 promoter binding, however. More significantly, assays using paired Birc5 (survivin) promoter-luciferase reporters with either wild-type or mutated Myb binding sites showed that FoxM1 was unable to bind and activate the promoter in the absence of B-Myb binding. Our data suggest that B-Myb is required as a pioneer factor to enable FoxM1 binding to G2/M gene promoters and explains how these transcription factors may collaborate to induce mitosis.

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