CELL-CYCLE-REGULATED TRANSCRIPTION OF THE CLB2 GENE IS DEPENDENT ON MCM1 AND A TERNARY COMPLEX FACTOR

Clb2 is the major B-type mitotic cyclin required for entry into mitosi s in the budding yeast Saccharomyces cerevisiae. We showed that accumu lation of CLB2 transcripts in G(2) cells is controlled at the transcri ptional level and identified a 55-bp upstream activating sequence (UAS ) containing an Mcm1 binding site as being necessary and sufficient fo r cell cycle regulation. Sequences within the cell cycle-regulated UAS were shown to bind Mcm1 in vitro, and mutations which abolished Mcm1- dependent DNA binding activity eliminated cell cycle-regulated transcr iption in vivo. A second protein with no autonomous DNA binding activi ty was also recruited into Mcm1-UAS complexes, generating a ternary co mplex. A point mutation in the CLB2 UAS which blocked ternary complex formation, but still allowed Mcm1 to bind, resulted in loss of cell cy cle regulation in vivo, suggesting that the ternary complex factor is also important in control of CLB2 transcription. We discuss the possib ility that the CLB2 gene is coregulated,vith other genes known to be r egulated with the same periodicity and suggest that Mcm1 and the terna ry complex factor may coordinately regulate several other G(2)-regulat ed transcripts.