PROPERTIES OF AN INTERGENIC TERMINATOR AND START SITE SWITCHING THAT REGULATES IMD2 TRANSCRIPTION IN YEAST

Department of Biochemistry, Emory University School of Medicine, Atlanta, GA 30322

^* To whom correspondence should be addressed. Email: dreines{at}emory.edu.

	Abstract

The IMD2 gene in Saccharomyces cerevisiae is regulated by intracellular guanine nucleotides. Regulation is exerted through the choice of alternative transcription start sites that results in synthesis of either an unstable short transcript terminating upstream of the start codon or a full length productive IMD2 mRNA. Start site selection is dictated by the intracellular guanine nucleotide levels. Here we have mapped the polyadenylation sites of the upstream, unstable short transcripts that form a heterogeneous family of RNAs of 200 nt. The switch from the upstream to downstream start sites required the Rpb9 subunit of RNA polymerase II. The enzyme's ability to locate the downstream initiation site decreased exponentially as the start was moved downstream from the TATA box. This suggests that RNA polymerase II's pincer grip is important as it slides on DNA in search of a start site. Exosome degradation of the upstream transcripts was highly dependent upon the distance between the terminator and the promoter. Similarly, termination was dependent upon the Sen1 helicase when close to the promoter. These findings extend the emerging concept that distinct modes of termination by RNA polymerase II exist and that the distance of the terminator from the promoter, as well as its sequence, is important for the pathway chosen.