Histone transcription in cell cycle mutants of fission yeast.

The level of histone H2B transcripts peak during S-phase of the fission yeast Schizosaccharomyces pombe. The pattern of transcript accumulation has been monitored in temperature-sensitive mutants which block at different times during the cell cycle, at start in G1 where the cell becomes committed to the mitotic cycle, in G1 after start, and during S-phase. Cells blocked before start using cdc10-129 do not accumulate histone H2B transcripts, but cells blocked after start using cdc22-C11 do show accumulation. Transcript levels increase in another mutant cdc20-M10 which also blocks in G1. These experiments establish that histone H2B transcripts increase in level in preparation for S-phase during late G1 before any DNA synthesis. Passage of start begins a sequence of events leading to S-phase which includes an increase in histone H2B transcript levels. In the cdc20 and cdc22 mutants transcript levels do not decrease normally suggesting that the signals which lead to the fall in level are not given in these G1-arrested cells. The mutants cdc17-K42 (defective in DNA ligase) and cdc24-M38 block in late S-phase after the DNA content has doubled. Histone H2B transcripts increase normally but remain at a high level. In these mutants even though DNA content has doubled, the mechanisms which lead to a fall in transcript levels appear not to be brought into play.