A NOVEL MCM1-DEPENDENT ELEMENT IN THE SWI4, CLN3, CDC6, AND CDC47 PROMOTERS ACTIVATES M G(1)-SPECIFIC TRANSCRIPTION/

We have identified a novel promoter element that confers M/G(1)-specif ic transcription in Saccharomyces cerevisiae. This element, which we c all an ECB (early cell cycle box), was first identified in the SWI4 pr omoter, but it is also present in the promoter of a G(1) cyclin CLN3, as well as in the promoters of three DNA replication genes: CDC6, CDC4 7, and CDC46. Transcripts from all five of these genes oscillate durin g the cell cycle and peak at the M/G(1) boundary, as do isolated ECB e lements in reporter constructs. The ECB element contains an Mcm1 bindi ng site to which Mcm1 binds in vitro, and an Mcm1-VP16 fusion, which p laces a constitutive activator on Mcm1-binding sites in vivo, can dere gulate ECB-containing promoters. Mcm1 is a transcription factor that i s also required for minichromosome maintenance. We provide evidence th at the replication defect of mcm1 mutants can be suppressed by ectopic CDC6 transcription. Periodic expression of SWI4 and CLN3 may be impor tant for cell cycle progression, as we find that these genes are both haploinsufficient and rate limiting for G(1) progression. We suggest t hat ECB-regulated gene products play critical roles in promoting the i nitiation of S-phase, both by regulating CLN1 and CLN2 transcription a nd as components of the initiation complex on origins of replication.