Break-induced replication and telomerase-independent telomere maintenance require Pol32

Abstract

Break-induced replication repair (BIR) of a DNA double-strand break requires Polα and Polδ to initiate replication, while Polɛ is needed to extend the replication patch. This BIR also uniquely requires Pol32, a Polδ subunit that is not necessary for normal replication. Break-induced replication (BIR) is an efficient homologous recombination process to initiate DNA replication when only one end of a chromosome double-strand break shares homology with a template1,2,3,4,5. BIR is thought to re-establish replication at stalled and broken replication forks and to act at eroding telomeres in cells that lack telomerase in pathways known as ‘alternative lengthening of telomeres’ (reviewed in refs 2, 6). Here we show that, in haploid budding yeast, Rad51-dependent BIR induced by HO endonuclease requires the lagging strand DNA Polα-primase complex as well as Polδ to initiate new DNA synthesis. Polε is not required for the initial primer extension step of BIR but is required to complete 30 kb of new DNA synthesis. Initiation of BIR also requires the nonessential DNA Polδ subunit Pol32 primarily through its interaction with another Polδ subunit, Pol31. HO-induced gene conversion, in which both ends of a double-strand break engage in homologous recombination, does not require Pol32. Pol32 is also required for the recovery of both Rad51-dependent and Rad51-independent survivors in yeast strains lacking telomerase. These results strongly suggest that both types of telomere maintenance pathways occur by recombination-dependent DNA replication. Thus Pol32, dispensable for replication and for gene conversion, is uniquely required for BIR; this finding provides an opening into understanding how DNA replication re-start mechanisms operate in eukaryotes. We also note that Pol32 homologues have been identified both in fission yeast and in metazoans where telomerase-independent survivors with alternative telomere maintenance have also been identified2,6,7.