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

Corresponding author: Raymund J. Wellinger, Faculté de Médecine, Université de Sherbooke, 3001 12 Ave. Nord, Sherbooke, Quebec J1H 5N4, Canada., raimund.wellinger{at}usherbrooke.ca (E-mail)

Communicating editor: L. PILLUS

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 rad27 cells. The results from studies of the replication intermediates of a linear plasmid in rad27 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.

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