Chromosomal ARS1 has a single leading strand start site

Initiation sites for DNA synthesis in the chromosomal autonomously replicat ing sequence (ARS)1 of Saccharomyces cerevisiae were detected at the nucleo tide level. The transition from discontinuous to continuous synthesis defin es the origin of bidirectional replication (OBR), which mapped adjacent to the origin recognition complex binding site. To ascertain which sites repre sented starts for leading or lagging strands, we characterized DNA replicat ion from ARS1 in a cdcs (DNA ligase I) mutant, defective for joining Okazak i fragments. Leading strand synthesis in ARS1 initiated at only a single si te, the OBR. Thus, replication in S. cerevisiae is not initiated stochastic ally by choosing one out of multiple possible sites but, rather, is a highl y regulated process with one precise start point.