EST1 HAS THE PROPERTIES OF A SINGLE-STRANDED TELOMERE END-BINDING PROTEIN

In Saccharomyces cerevisiae, deletion of the EST1 gene results in phen otypes identical to those displayed by a deletion of a known component of telomerase (the yeast telomerase RNA), arguing that EST1 is also c ritical far telomerase function. In this study, we show that the Est1 protein binds to yeast G-rich telomeric oligonucleotides in vitro. Bin ding is specific for single-stranded substrates and requires a free 3' terminus, consistent with the properties expected for a protein bound to the 3' single-stranded G-rich extension present at the telomere. A ssessment of the in vivo function of this single-stranded DNA-binding protein has shown that EST1 acts in the same pathway of telomere repli cation as the TLC1 telomerase RNA, by several different genetic criter ia: est1 tlc1 double mutant strains show no enhancement of phenotype r elative to either single mutant strain, and EST1 dominant mutations ha ve an effect on telomeric silencing similar to that displayed by TLC1 previously. We propose that Est1 is a telomere end-binding protein tha t is required to mediate recognition of the end of the chromosome by t elomerase.