An S-phase checkpoint gated by Bms1 and Ttf1 at the rDNA loci

During S-phase, active rDNA transcription conflicts with rDNA replication along hundreds of copies of tandemly arrayed rDNA genes. Ttf1 prevents conflicting collisions by binding to specific 'Sal-boxes', thus forming replication-fork-barriers. However, understanding of how Ttf1 is displaced from 'Sal-boxes' to allow replication-fork progression remains elusive. Here we report that Bms1, a nucleolar GTPase, interacts with Ttf1 to disassociate the Ttf1-DNA complex. GTPase-activity compromised zebrafish Bms1 mutants display 'Sal-boxes' accumulation of Ttf1 and resultant replication-fork stall. However, genomic DNA undergo over-replication that in turn activates DNA-damage response and arrests cell cycle at the S-phase, causing a small liver in bms1 mutants. We propose that Ttf1 and Bms1 together impose a nucleolar checkpoint on cell cycle progression through balancing S-phase rDNA transcription and replication. Overall design: Examination of Ttf1 binding site in 3dpf wildtype zebrafish embryo