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Abstract

Cells in the double-stranded replicative form DNA multiplication stage of the infection with bacteriophage φX174 were made permeable to nucleotides and the flow of dBrUTP density label into the RF‡ of the cells was followed. It is concluded that there is a major process of RF synthesis in the permeable cells which gives rise to half-synthetic RF (one new, one conserved strand) and a minor process which gives rise to RF in which the fraction of newly synthesized DNA is only about 5%. The rate of chain elongation in the major process is approximately 200 nucleotides/sec at 25 °C and there is infectivity connected with the newly synthesized strands. RF with new complementary strand accumulates more rapidly than RF with new viral strand during the early synthesis. In order to explain this and other results, it is proposed that intermediates of in vivo replication continue to replicate in the permeable cells and that the viral strand but not the complementary strand is initiated in some of these in vivo intermediates. The opposite is not thought to occur. 5% synthetic RF is thought to result from the filling of natural gaps in RFII synthesized in vivo. This RF is made both in polA− cells and in polA+ cells and is labelled at the 3′-hydroxyl end of a DNA chain.