THE CLN3 CYCLIN IS DOWN-REGULATED BY TRANSLATIONAL REPRESSION AND DEGRADATION DURING THE G(1) ARREST CAUSED BY NITROGEN DEPRIVATION IN BUDDING YEAST

Nutrients are among the most important trophic factors in all organism s, When deprived of essential nutrients, yeast cells use accumulated r eserves to complete the current cycle and arrest in the following G(1) phase, We show here that the Cln3 cyclin, which has a key role in the timely activation of SBF (Swi4-Swi6)- and MBF (Mbp1-Swi6)-dependent p romoters in late G(1), is down-regulated rapidly at a post-transcripti onal level in cells deprived of the nitrogen source, In addition to th e fact that Cln3 is degraded faster by ubiquitin-dependent mechanisms, we have found that translation of the CLN3 mRNA is repressed similar to 8-fold under nitrogen deprivation conditions, As a consequence, bot h SBF- and MBF-dependent expression is strongly down-regulated, Mainly because of their transcriptional dependence on SBF, and perhaps with the contribution of similar post-transcriptional mechanisms to those f ound for Cln3, the G(1) cyclins Cln1 and 2 become undetectable in star ved cells. The complete loss of Cln cyclins and the sustained presence of the Clb-cyclin kinase inhibitor Sic1 in starved cells may provide the molecular basis for the G(1) arrest caused by nitrogen deprivation .