CELL-CYCLE, DNA-DAMAGE AND HEAT-SHOCK REGULATE SUC22+ EXPRESSION IN FISSION YEAST

The suc22(+) gene of Schizosaccharomyces pombe encodes the small subun it of ribonucleotide reductase. Two transcripts that hybridise to suc2 2(+) have previously been described: a constitutive transcript of 1.5 kb, and a transcript of similar to 1.9 kb that is induced when DNA rep lication is blocked by hydroxyurea. In this paper we show that both tr anscripts derive ii-out the suc22(+) gene, are polyadenylated, and hav e transcription initiation sites separated by similar to 550 nucleotid es. The absence of translation initiation codons and predicted intron splice sites within this 550 nucleotide region suggests strongly that both transcripts encode the same protein. Under normal growth conditio ns. the larger suc22(+) transcript is present at a very low level. Thi s low level expression is periodic during the cell cycle, showing a pa ttern similar to that of other genes under regulation by MCB elements with a maximum in G1/S phase. Consistent with this, there are MCB elem ents upstream or the initiation site of the transcript. This pattern o f expression contrasts with the continuous expression, at a much highe r level, of the smaller suc22(+) transcript. The larger suc22(+) trans cript is induced by exposure of cells to 4-nitroquinoline oxide (4-NQO ), a UV-mimetic agent that causes DNA damage. The transcriptional resp onse to 4-NQO is observed in cells previously arrested in G2 by a cdc2 (ts) mutation, demonstrating that induction can occur outside S phase. We show that the radl(+) gene, part of the mitotic checkpoint, is req uired for induction of the large transcript. Exposure of cells to heat shock also induces the suc22(+) large transcript: a consensus heat sh ock element has been identified upstream of the large transcript start site.