Feedback regulation of the MBF transcription factor by cyclin Cig2.

The Mlu1-binding factor (MBF) from the fission yeast Schizosaccharomyces pombe contains the proteins Res1p and Res2p and binds to the Mlu1 cell-cycle box (MCB) element in DNA, activating the transcription of genes required for S phase. We report here that the cell-cycle-regulated expression of the cyclin cig2 gene is dependent on MBF. Deletion of MCB elements in the cig2 promoter perturbed the expression not only of cig2 but also of other MBF-dependent genes, indicating that Cig2p could regulate MBF activity. Cig2p can bind to Res2p, promote the phosphorylation of Res1p and inhibit MBF-dependent gene transcription. Cig2p thus forms an autoregulating feedback-inhibition loop with MBF which is important for normal regulation of the cell cycle.