DNA Polymerase α (swi7) and the Flap Endonuclease Fen1 (rad2) Act Together in the S-Phase Alkylation Damage Response in S. pombe

Polymerase a is an essential enzyme mainly mediating Okazaki fragment synthesis during lagging strand replication. A specific point mutation in Schizosaccharomyces pombe polymerase alpha named swi7-1, abolishes imprinting required for mating-type switching. Here we investigate whether this mutation confers any genome-wide defects. We show that the swi7-1 mutation renders cells hypersensitive to the DNA damaging agents methyl methansulfonate (MMS), hydroxyurea (HU) and UV and incapacitates activation of the intra-S checkpoint in response to DNA damage. In addition we show that, in the swi7-1 background, cells are characterized by an elevated level of repair foci and recombination, indicative of increased genetic instability. Furthermore, we detect novel Swi1-, -Swi3- and Pol alpha- dependent alkylation damage repair intermediates with mobility on 2D-gel that suggests presence of single-stranded regions. Genetic interaction studies showed that the flap endonuclease Fen1 works in the same pathway as Pol alpha in terms of alkylation damage response. Fen1 was also required for formation of alkylation-damage specific repair intermediates. We propose a model to explain how Pol alpha, Swi1, Swi3 and Fen1 might act together to detect and repair alkylation damage during S-phase.