THE UBIQUITIN-CONJUGATING ENZYME RAD6 (UBC2) IS REQUIRED FOR SILENCING IN SACCHAROMYCES-CEREVISIAE

It has been previously shown that genes transcribed by RNA polymerase II (RNAP II) are subject to position effect variegation when located n ear yeast telomeres, This telomere position effect requires a number o f gene products that are also required for silencing at the HML and HM R loci, Here, rye show that a null mutation of the DNA repair gene RAD 6 reduces silencing of the HM loci and lowers the mating efficiency of MATa strains, Likewise, rad6-Delta reduces silencing of the telomere- located RNAP II-transcribed genes URA3 and ADE2, We also show that the RNAP III-transcribed tyrosyl tRNA gene, SUP4-o, is subject to positio n effect variegation when located near a telomere and that this silenc ing requires the RAD6 and SIR genes. Neither of the two known Rad6 bin ding factors, Rad18 and Ubr1, is required for telomeric silencing, Sin ce Ubr1 is the recognition component of the N-end rule-dependent prote in degradation pathway, this suggests that N-end rule-dependent protei n degradation is not involved in telomeric silencing, Telomeric silenc ing requires the amino terminus of Rad6. Two rad6 point mutations, rad 6(C88A) and rad6(C88S), which are defective in ubiquitin-conjugating a ctivity fail to complement the silencing defect, indicating that the u biquitin-conjugating activity of RAD6 is essential for full telomeric silencing.