bThere are nearly 50 forkhead (FOX) transcription factors encoded in the human genome and, due to sharing a common DNA binding domain, they are all thought to bind to similar DNA sequences. It is therefore unclear how these transcription factors are targeted to specific chromatin regions to elicit specific biological effects. Here, we used chromatin immunoprecipitation followed by sequencing (ChIP-seq) to investigate the genome-wide chromatin binding mechanisms used by the forkhead transcription factor FOXM1. In keeping with its previous association with cell cycle control, we demonstrate that FOXM1 binds and regulates a group of genes which are mainly involved in controlling late cell cycle events in the G 2 and M phases. However, rather than being recruited through canonical RYAAAYA forkhead binding motifs, FOXM1 binding is directed via CHR (cell cycle genes homology region) elements. FOXM1 binds these elements through protein-protein interactions with the MMB transcriptional activator complex. Thus, we have uncovered a novel and unexpected mode of chromatin binding of a FOX transcription factor that allows it to specifically control cell cycle-dependent gene expression.
There are nearly 50 different forkhead transcription factors encoded in mammalian genomes, and these proteins all contain the conserved forkhead DNA binding domain (reviewed in references 1 and 2). Forkhead transcription factors are involved in controlling a wide range of biological processes and are aberrantly expressed or regulated in disease states, including cancer (reviewed in reference 2). However, due to sharing a common DNA binding domain, forkhead transcription factors are generally believed to bind to variations of the RYAAAYA motif. Hence, it is unclear how individual forkhead proteins are specifically recruited to the regulatory regions of different cohorts of target genes to control defined biological responses. One key process which is controlled by forkhead transcription factors is the cell cycle and, in particular, the G 2 -M transition. The initial links to G 2 -M control were made with the Saccharomyces cerevisiae forkhead protein Fkh2, which controls the temporal expression of a cluster of genes at this phase of the cell cycle (reviewed in reference 3). More recently, members of the FOXO and FOXM classes of forkhead transcription factors have been linked with controlling the same process in mammalian cells (4-6). In both cases, forkhead transcription factors coordinate the integration of signals from the cell cycle regulatory machinery to transcriptional outputs. This is exemplified by the links to the cell cycle regulated Polo-like kinase PLK1, which is recruited to cell cycle-regulated promoters through promoter elements bound by the forkhead transcription factors FOXM1 and Fkh2, albeit indirectly in the case of Fkh2 (7,8).In mammalian cells, the transcriptional control of a cluster of genes at the G 2 -M transition, is coordinated through promoter elements which typically contain CHR (cell cycle genes homology region) and...