ENHANCEMENT OF TELOMERE-PLASMID SEGREGATION BY THE X-TELOMERE ASSOCIATED SEQUENCE IN SACCHAROMYCES-CEREVISIAE INVOLVES SIR2, SIR3, SIR4 AND ABF1

We have previously shown that circular replicating plasmids that carry yeast telomere repeat sequence (TG(1-3)) tracts segregate efficiently relative to analogous plasmids lacking the TG(1-3) tract and this efficient segregation is dependent upon RAP1. While a long TG(1-3) tract is sufficient to improve plasmid segregation, the segregation efficiency of telomere plasmids (TEL-plasmids) is enhanced when the X-Telomere Associated Sequence (X-TAS) is also included on the plasmids. We now demonstrate that the enhancement of TEL-plasmid segregation by the X-TAS depends on SIR2, SIR3, SIR4 and ABF1 in trans and requires the Abflp-binding site within the X-TAS. Mutation of the Abflp-binding site within the X-TAS results in TEL-plasmids that are no longer affected by mutations in SIR2, SIR3 or SIR4, despite the fact that other Abflp-binding sites are present on the plasmid. Mutation of the ARS consensus sequence within the X-TAS converts the X-TAS from an enhancer element to a negative element that interferes with TEL-plasmid segregation in a SIR-dependent manner. Thus, telomere associated sequences interact with TG(1-3) tracts on the plasmid, suggesting that the TASs have an active role in modulating telomere function.