Periodic transcription as a means of regulating gene expression during the cell cycle: contrasting modes of expression of DNA ligase genes in budding and fission yeast.

Using cultures synchronised by three independent procedures, we have shown that the CDC9 gene, coding for DNA ligase, is periodically expressed in the Saccharomyces cerevisiae cell cycle. The level of CDC9 transcript increases many fold in late G1 reaching a peak at about the G1/S phase boundary and preceding the peak in histone message by some 20 min. The level of DNA ligase itself also fluctuates, showing the expected pattern for a stable enzyme synthesised periodically. In contrast, the transcript from the DNA ligase gene (CDC17) of Schizosaccharomyces pombe is present at a constant level throughout the cell cycle, and no fluctuation in amount was detected, although the histone H2A showed the expected periodic synthesis. Furthermore, DNA ligase activity remains at a constant level during the S. pombe cell cycle showing that there is unlikely to be any form of translational control. These contrasting modes of expression of the DNA ligase genes in the two organisms suggests that when periodic transcription is observed from an essential cell cycle gene, it may have no particular significance for regulating progress through the cell cycle. Also, regulatory circuits may be less well conserved between organisms than the processes they control and thus different organisms may utilise quite different modes of control to achieve the same ends.