Feedback regulation of the MBF transcription factor by cyclin Cig2

The Mlu1-binding factor (MBF) from the fission yeast Schizosaccharomyces po mbe contains the proteins Res1p and Res2p and binds to the Mlu1 cell-cycle box (MCB) element in DNA, activating the transcription of genes required fo r S phase. We report here that the cell-cycle-regulated expression of the c yclin cig2 gene is dependent on MBF Deletion of MCB elements in the cig2 pr omoter perturbed the expression not only of cig2 but also of other MBF-depe ndent genes, indicating that Cig2p could regulate MBF activity. Cig2p can b ind to Res2p, promote the phosphorylation of Res1p and inhibit MBF-dependen t gene transcription. Cig2p thus forms an autoregulating feedback-inhibitio n loop with MBF which is important for normal regulation of the cell cycle.