A replication stress safeguard provided by the Elg1 Replication Factor C-like complex

The Elg1 Replication Factor C-like complex (Elg1-RLC) that functions as a proliferating cell nuclear antigen (PCNA) unloader, is known to be involved in multiple DNA replication/repair-related activities from yeast to humans. By exploiting disassembly prone PCNA mutants, we reveal that Elg1-RLC uses its PCNA unloading activity to counter the DNA-alkylating agent methyl-methanesulfonate (MMS)-mediated slow progression of replication forks (RFs). Despite having a largely functional DNA damage response (DDR), the viability loss of elg1Δ-DDR double mutants, in the presence of MMS, matches that of mec1Δ and rad53Δ cells, deficient for the central checkpoint kinases. This suggests that elg1Δ-DDR double mutants experience RF collapse when exposed to MMS. Indeed, in response to MMS, accumulation of Rad52 foci in the replicative elg1Δ-DDR cells supports this possibility. However, the failure of rescuing elg1Δ-DDR mutants by elevating deoxynucleoside triphosphate (dNTP) levels (by deleting the ribonucleotide reductase SML1) eliminates the possibility of a Rad53-regulated dNTP shortage-mediated fork collapse. Thus, we propose an S-phase checkpoint regulatory role of Elg1-RLC that works through a noncanonical pathway parallel to the canonical one. Collectively, our findings suggest a model in which Elg1-RLC, by timely unloading chromatin-bound PCNA from the damaged/stalled forks, coordinates the DDR pathways to safeguard the integrity of RFs under replication stress.