AN IMPAIRED RNA POLYMERASE-II ACTIVITY IN SACCHAROMYCES-CEREVISIAE CAUSES CELL-CYCLE INHIBITION AT START

Saccharomyces cerevisiae cells harboring the temperature-sensitive mut ation rpo21-4, in the gene encoding the largest subunit of RNA polymer ase II, were shown to be partially impaired for cell-cycle progress at a permissive temperature, and to become permanently blocked at the ce ll-cycle regulatory step, START, at a restrictive temperature. The rpo 21-4 mutation was lethal in combination with cdc28 mutations in the p3 4 protein kinase gene required for START. Transcripts of the CLN1 and CLN2 genes, encoding G1-cyclin proteins that, along with p34, are nece ssary for START, were decreased in abundance by the rpo21-4 mutation a t a restrictive temperature. Increased G1-cyclin production, by expres sion of the CLN1 or CLN2 genes from a heterologous GAL promoter, overc ame the rpo21-4 - mediated START inhibition, but such mutant cells nev ertheless remained unable to proliferate at a restrictive temperature. These findings reveal that START can be particularly sensitive to an impaired RNA polymerase II function, presumably through effects on G1- cyclin expression.