Genes for a Nuclease and a Protease Are Involved in the Drastic Decrease in Cellular RNA Amount in Fission Yeast Cells during Nitrogen Starvation.

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Cellular RNA in Schizosaccharomyces pombe cells drastically decreases in amount during nitrogen starvation. Previously, we found and purified a soluble RNA-degrading enzyme whose activity drastically increased in the cells of S. pombe undergoing nitrogen starvation. The enzyme was a nuclease encoded by pnu1+. In this study, the increase in the RNA-degrading activity and the decrease in cellular RNA level are examined in a null-mutant of pnu1+ (pnu1Δ). During nitrogen starvation, wild-type cells show an apparent increase in RNA-degrading activity, whereas the pnu1Δ cells do not. The wildtype cells show a drastic decrease in cellular RNA amount, whereas the pnu1Δ cells show only a slight decrease. These results suggest that Pnu1 nuclease is implicated in the decrease in cellular RNA amount during nitrogen starvation, probably via the RNA-degrading activity. The increase in the RNA-degrading activity is independent of both the Wis1 stress-activated MAP kinase cascade and Tor1 signaling pathway, but it is strongly dependent on isp6+, a gene for a possible protease, whose expression is induced during nitrogen starvation. A disruption mutant for isp6+ (isp6Δ) is deficient in both the increase in the RNA-degrading activity and the drastic decrease in the cellular RNA amount during nitrogen starvation, which suggests that isp6+ is involved in the RNA degradation via regulating the RNA-degrading activity of Pnu1.
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