Abstract
Replicative intermediates of M2 DNA, pulse-labeled with [3H]-thymidine, were isolated and the mode of its replication was examined. Compared with mature M2 DNA the replicative intermediates had the following properties: i) higher sedimentation coefficient in sucrose gradient, ii) higher buoyant density in CsCl equilibrium density gradient, and iii) sensitivity to nuclease Sl. Two types of intermediates were found by electron microscopic observation. One is unit-length duplex molecules with a single-stranded branch of various lengths. The other is unit-length linear molecules consisting of one single-stranded and one double- stranded region of variable proportions. Distribution of the radioactivity in pulse-labeled completed molecules was analyzed with restriction endonucleases. Both ends of the genome had higher radioactivity, showing that the ends are termini of replication. These results indicate that the replication of M2 DNA proceeds from each end by displacing parental single-stranded DNA.
