Differential Processing of Leading- and Lagging-Strand Ends at Saccharomyces cerevisiae Telomeres Revealed by the Absence of Rad27p Nuclease

Saccharomyces cerevisiae strains lacking the Rad27p nuclease, a homolog of the mammalian FEN-1 protein, display an accumulation of extensive single-stranded G-tails at telomeres. Furthermore, the lengths of telomeric repeats become very heterogeneous. These phenotypes could be the result of aberrant Okazaki fragment processing of the C-rich strand, elongation of the G-rich strand by telomerase, or an abnormally high activity of the nucleolytic activities required to process leading-strand ends. To distinguish among these possibilities, we analyzed strains carrying a deletion of the RAD27 gene and also lacking genes required for in vivo telomerase activity. The results show that double-mutant strains died more rapidly than strains lacking only telomerase components. Furthermore, in such strains there is a significant reduction in the signals for G-tails as compared to those detected in rad27Δ cells. The results from studies of the replication intermediates of a linear plasmid in rad27Δ cells are consistent with the idea that only one end of the plasmid acquires extensive G-tails, presumably the end made by lagging-strand synthesis. These data further support the notion that chromosome ends have differential requirements for end processing, depending on whether the ends were replicated by leading- or lagging-strand synthesis.