AN SSN6-TUP1-DEPENDENT NEGATIVE REGULATORY ELEMENT CONTROLS SPORULATION-SPECIFIC EXPRESSION OF DIT1 AND DIT2 IN SACCHAROMYCES-CEREVISIAE

Sporulation of the yeast Saccharomyces cerevisiae is a process of cell ular differentiation that occurs in MATa/MAT alpha diploid cells in re sponse to starvation. The sporulation-specific genes DIT1 and DIT2, wh ich are required for spore wall formation, are activated midway throug h the sporulation program, with maximal transcript accumulation occurr ing at the time of prospore enclosure, In this study, we have identifi ed a negative regulatory element, termed NRE(DIT), that is located bet ween the start sites of transcription of these divergently transcribed genes. This element, which prevents expression of the DIT1 and DIT2 g enes during vegetative growth, reduces expression of a CYC1-lacZ repor ter gene more than 1,000-fold and acts in an orientation- and position -independent manner. We found that the ability of NRE(DIT) to turn off expression of the reporter gene and the chromosomal DIT1 and DIT2 gen es in vegetative cells requires the Ssn6-Tup1 repression complex. Inte restingly, NRE(DIT)-mediated repression of the reporter gene is mainta ined during sporulation, Derepression during sporulation requires comp lex interactions among several cis-acting elements. These are present on an similar to 350-bp DNA fragment extending from NRE(DIT) to the TA TA box and an similar to 125-bp fragment spanning the TATA box of DIT1 . Additionally, a region of NRE(DIT) which is very similar in sequence to UAS(SPS4), an element that activates gene expression midway throug h sporulation, contributes both to vegetative repression and to sporul ation-specific induction of DIT1. We propose a model to explain the re quirement for multiple elements in overcoming NRE(DIT)-mediated repres sion during sporulation.