Importancia de la fase g1 del ciclo celularen medios pobres en nitrógeno en schizosaccharomyces pombe

Resumen

The fission yeast Schizosaccharomyces pombe is an ideal model to study cell cycle progression under different growth conditions. In nitrogen rich medium, cells show a very short G1 phase and the cell mass control occurs before mitosis. However, under nitrogen limiting conditions, cells undergo mitosis at a reduced cell size and enlarge the G1 phase to reach the necessary cell mass for the S-phase onset. Rum1 and Ste9 are two proteins that play a major role in enlarging the G1 phase. Rum1 is a CDK inhibitor that negatively regulates CDK-cyclin complexes, whereas Ste9 is an activator of the APC/C (Anaphase Promoting Complex/Cyclosome). Exponentially growing fission yeast cells lacking rum1 and ste9 are viable and do not show any apparent phenotype when they are grown in nitrogen rich medium. However, under reduced nitrogen conditions (minimal medium with phenylalanine) these cells show premature entry into S-phase, spend more time in S-phase and require the DNA damage checkpoint to survive. Indeed, rum1∆ ste9∆ cells showed an accumulation of DNA damage in this medium and the percentage of cells elongated and arrested in G2 increased over time. An increase in the G2 phase rescues the DNA damage of rum1∆ ste9∆ cells, whereas shortening G2 by deleting Ppa2 (encoding one of the two catalytic subunits of PP2A) results in lower viability. Interestingly, deletion of the G1 cyclin Cig2 not only did not rescue the elongation phenotype but it made it worse. The premature S-phase entry in rum1∆ ste9∆ cells is accompanied by a deregulated MBF activity, which is the transcription factor responsible for the transcription of a set of S-phase genes. It has been described that Cdc2 plays a role in MBF activation in late G1. In summary, we demonstrate that a proper regulation of MBF activity and low CDK activity are crucial to avoid DNA damage in nitrogen-poor medium.