Origin and direction of synthesis of bacteriophage fl DNA.

The origin and direction of synthesis in vivo of the viral and complementary strands of f1 DNA were studied by measuring the distribution of radioactivity along the genome after a short pulse of [3H]thymidine. The results indicate that the origins of replication of viral and complementary strands are located close to one another, probably both within a restriction fragment (HaeIII-G) which is about 120 bases long. Replication of both viral and complementary strands proceeds in the 5' leads to 3' overall direction. Thus, the viral strand is elongated in the counterclockwise and the complementary strand in the clockwise direction on the standard genetic map. A model is proposed in which only two mechanisms are invoked to generate all f1 DNA species: (1) the conversion of single-stranded viral DNA into double-stranded molecules and (2) the synthesis of viral single strands from double strands.