A MEC1-DEPENDENT AND RAD53-DEPENDENT CHECKPOINT CONTROLS LATE-FIRING ORIGINS OF DNA-REPLICATION

DNA replication in eukaryotic cells initiates from many replication or igins(1) which fire throughout the S phase of the cell cycle in a pred ictable pattern: some origins fire early, others late(2). Little is kn own about how the initiation of DNA replication and the elongation of newly synthesized DNA strands are coordinated during S phase. Here we show that, in budding yeast, hydroxyurea, which blocks the progression of replication forks from early-firing origins, also inhibits the fir ing of late origins. These late origins are maintained in the initiati on-competent prereplicative state for extended periods. The block to l ate origin firing is an active process and is defective in yeast with mutations in the rad53 and mec1 checkpoint genes, indicating that regu lation of late origin firing may also be an important component of the 'intra-S-phase' checkpoint(3) and may aid cell survival under adverse conditions.