Abstract
Sgs1 is a yeast DNA helicase functioning in DNA replication and repair, and is the orthologue of the human Bloom’s syndrome helicase BLM. Here we analyze the mutation signature associated with SGS1 deletion in yeast, and find frequent copy number changes flanked by regions of repetitive sequence and high R-loop forming potential. We show that loss of SGS1 increases R-loop accumulation and sensitizes cells to replication-transcription collisions. Accordingly, in sgs1Δ cells the genome-wide distribution of R-loops shifts to known sites of Sgs1 action, replication pausing regions, and to long genes. Depletion of the orthologous BLM helicase from human cancer cells also increases R-loop levels, and R-loop-associated genome instability. In support of a direct effect, BLM is found physically proximal to DNA:RNA hybrids in human cells, and can efficiently unwind R-loops in vitro. Together our data describe a conserved role for Sgs1/BLM in R-loop suppression and support an increasingly broad view of DNA repair and replication fork stabilizing proteins as modulators of R-loop mediated genome instability.
