DNA-damage dependent association of Orc6 to ISWI during S-phase modulates chromatin remodeling [CUT_and_RUN]

The human Origin Recognition Complex subunit 6 has recently garnered significant attention because of its dispensability in the licensing of origins. The loss of human Orc6 doesn’t impact MCM loading in vivo, while in vitro reconstitution experiments have demonstrated that MCM loading can occur without Orc6, but could be stimulated upon the addition of Orc6. We recently reported an unexpected role for hOrc6, in promoting S-phase progression post pre-RC assembly and DNA damage response (DDR). Using CUT&RUN to map the binding of Orc6 genome-wide, we find that Orc6 is present at replication origins and at sites of DNA damage during S-phase. Upon encountering oxidative damage in S-phase, Orc6 associates with chromatin remodelers, SMARCA1/SNF2L, also a component of the replication fork. Loss of Orc6 shows enhanced genome-wide association of SMARCA1 to Orc6-bound sites, concomitant with chromatin remodeling at these sites supporting the model that Orc6 is a negative regulator of SMARCA1 and constrains its localization. Upon DNA damage, enhanced association of Orc6 to SMARCA1 prevents chromatin remodeling activity in order to halt replication progression. We propose that during S-phase, Orc6 is a barrier factor, which limits the activity of chromatin remodelers at replication and repair sites.