Cell cycle, DNA damage and heat shock regulatesuc22 ⁺ expression in fission yeast

Abstract

Thesuc22 ⁺ gene ofSchizosaccharomyces pombe encodes the small subunit of ribonucleotide reductase. Two transcripts that hybridise tosuc22 ⁺ have previously been described: a constitutive transcript of 1.5 kb, and a transcript of ∼1.9 kb that is induced when DNA replication is blocked by hydroxyurea. In this paper we show that both transcripts derive from thesuc22 ⁺ gene, are polyadenylated, and have transcription initiation sites separated by ∼550 nucleotides. 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 conditions, the largersuc22 ⁺ transcript is present at a very low level. This low level expression is periodic during the cell cycle, showing a pattern similar to that of other genes under regulation by MCB elements with a maximum in G1/S phase. Consistent with this, there are MCB elements upstream of the initiation site of the transcript. This pattern of expression contrasts with the continuous expression, at a much higher level, of the smallersuc22 ⁺ transcript. The largersuc22 ⁺ transcript is induced by exposure of cells to 4-nitroquinoline oxide (4-NQO), a UV-mimetic agent that causes DNA damage. The transcriptional response to 4-NQO is observed in cells previously arrested in G2 by acdc2 ^ts mutation, demonstrating that induction can occur outside S phase. We show that therad1 ⁺ gene, part of the mitotic checkpoint, is required for induction of the large transcript. Exposure of cells to heat shock also induces thesuc22 ⁺ large transcript: a consensus heat shock element has been identified upstream of the large transcript start site.