IDENTIFICATION OF HIGH-COPY DISRUPTERS OF TELOMERIC SILENCING IN SACCHAROMYCES-CEREVISIAE

The ends of chromosomes in Saccharomyces cerevisiae initiate a repress ive chromatin structure that spreads internally and inhibits the trans cription of nearby genes, a phenomenon termed telomeric silencing. To investigate the molecular basis of this process, we carried out a gene tic screen to identify genes whose overexpression disrupts telomeric s ilencing. We thus isolated 10 DOT genes (disruptor of telomeric silenc ing). Among these were genes encoding chromatin component Sir4p, DNA h elicase Dna2p, ribosomal protein L32, and two proteins of unknown func tion, Asf1p and Ifh1p. The collection also included genes that had not previously been identified: DOT1, DOT4, DOT5, DOT6, and TLC1, which e ncodes the RNA template component of telomerase. With the exception of TLC1, all these genes, particularly DOT1 and DOT4 also reduced silenc ing at other repressed loci (HM loci and rDNA) when overexpressed. Mor eover, deletion of the latter two genes weakened silencing as well, su ggesting that DOT1 and DOT4 normally play important roles in gene repr ession. DOT1 deletion also affected telomere tract length. The functio n of Dot1p is not known. The sequence of Dot4p suggests that it is a u biquitin-processing protease. Taken together, the DOT genes include bo th components and regulators of silent chromatin.