A Role for the Transcription Factors Mbp1 and Swi4 in Progression from G1 to S Phase

In budding yeast genes that encode G1 cyclins and proteins involved in DNA synthesis are transcriptionally activated in late G1. A transcription factor, called SBF, is composed of Swi4 and Swi6 proteins and activates transcription of G1 cyclin genes. A different, but related, complex called MBF binds to MCB elements (Mlu I cell cycle box) found in the promoter of most DNA synthesis genes. MBF contains Swi6 and a 120-kilodalton protein (p120). MBF was purified and the gene encoding p120 (termed MBP1) was cloned. A deletion of MBP1 was not lethal but led to deregulated expression of DNA synthesis genes, indicating a direct regulatory role for MBF in MCB-driven transcription. Mbp1 is related to Swi4. Strains deleted for both MBP1 and SWI4 were inviable, demonstrating that transcriptional activation by MBF and SBF has an important role in the transition from G1 to S phase.