A 15-BASE-PAIR ELEMENT ACTIVATES THE SPS4 GENE MIDWAY THROUGH SPORULATION IN SACCHAROMYCES-CEREVISIAE

Sporulation of the yeast Saccharomyces cerevisiae represents a simple developmental process in which the events of meiosis and spore wall fo rmation are accompanied by the sequential activation of temporally dis tinct classes of genes. In this study, we have examined expression of the SPS4 gene, which belongs to a group of genes that is activated mid way through sporulation. We mapped the upstream boundary of the regula tory region of SPS4 by monitoring the effect of sequential deletions o f 5'-flanking sequence on expression of plasmid-borne versions of SPS4 introduced into a MATa/MAT alpha Delta sps4/Delta sps4 strain. This a nalysis indicated that the 5' boundary of the regulatory region was wi thin 50 bp of the putative TATA box of the gene. By testing various ol igonucleotides that spanned this boundary and the downstream sequence for their ability to activate expression of a heterologous promoter, w e found that a 15-bp sequence sufficed to act as a sporulation-specifi c upstream activation sequence. This 15-bp fragment, designated UAS(SP S4), activated expression of a CYC1-lacZ reporter gene midway through sporulation and was equally active in both orientations. Extending the UAS fragment to include the adjacent 14-bp enhanced its activity 10-f old. We show that expression of SPS4 is regulated in a manner distinct from that of early meiotic genes: mutation of UME6 did not lead to ve getative expression of SPS4, and sporulation specific expression was d elayed by mutation of IME2. In vivo and in vitro assays suggested that a factor present in vegetative cells binds to the UAS(SPS4) element. We speculate that during sporulation this factor is modified to serve as an activator of the SPS4 gene or, alternatively, that it recruits a n activator to the promoter.