CELL CYCLE-DEPENDENT TRANSCRIPTION OF CLN1 INVOLVES SWI4 BINDING TO MCB-LIKE ELEMENTS

Two promoter elements have been defined that activate G(1)/S-specific transcription in Saccharomyces cerevisiae. SCB elements (CACGAAA) are activated by the Swi4-Swi6 complex, and RICH elements (ACGCGTNA) are a ctivated by the Mbp1-Swi6 complex. CLN1 encodes a cyclin which is expr essed during this interval, and requires Swi4 and Swi6 for peak transc ription, but it has no consensus SCB elements in its promoter. Two SCB -like sequences had been previously noted and suggested to be the func tional promoter elements. Our studies indicate that these sequences ar e unable to activate transcription of a lacZ reporter construct, or to bind Swi4-Swi6 complexes in vitro, However, a cluster of three sequen ces resembling MCB sequences are active promoter elements, sufficient to confer G(1)/S-specific transcription to a reporter. These sites are the predominant activation elements in the CLN1 promoter, and despite their resemblance to MCB elements, they bind Swi4-Swi6 complexes in v itro and require Swi4 and Swi6 for their activity in vivo. This indica tes that the sequences that promote Swi4/Swi6 binding have not been fu lly defined, or that there are multiple Swi4- and Swi6-containing comp lexes with distinct DNA binding specificities. In addition to these no vel Swi4/Swi6-binding sites, these studies also show that there must b e at least one novel promoter element that can confer G(1)/S-specific transcription to CLN1, because when all the potential SCB- and MCB-lik e sequences are eliminated the transcript is still cell cycle regulate d.