Multiple initiation sites of DNA replication flanking the origin region of λdv genome

Early replicative intermediates of λdv plasmid were prepared by an in vitro replication system in the presence of 2',3'-dideoxycytidine 5'-triphosphate, an inhibitor of DNA chain elongation. Short-chain DNAs produced from regions near the replication origin were purified from the intermediates. A fraction of the DNAs was covalently linked to primer RNA. The transition sites from primer RNA to DNA synthesis were mapped along the nucleotide sequence of the genome, by eliminating the RNA by alkaline hydrolysis and labeling the freshly exposed 5' ends of DNA with ³²P. The transition sites were found to be located on both sides of the ori region, which includes four 19-base-pair repeats where one of the λ specific initiator proteins, O, binds. No transition arose within the ori region. The transition sites are multiple on both sides of the ori region and are clustered in one of the two strands in such a way that DNA syntheses from the two sides converge. The frequency of the 'leftward' DNA synthesis is several times higher than that of 'rightward' synthesis, reflecting the asymmetric bidirectional replication of λdv DNA.