Regulation of coliphage f1 single-stranded DNA synthesis by a DNA-binding protein

Abstract

Control of single-strand DNA synthesis in coliphage f1 was studied with the use of mutants which are temperature sensitive in gene 2, a gene essential for phage DNA replication. Cells were infected at a restrictive temperature with such a mutant, and the DNA synthesized after a shift to permissive temperature was examined. When cells were held at 42 °C for ten or more minutes after infection, only single-stranded DNA was synthesized immediately after the shift to permissive temperature. This indicated that the accumulation of a pool of double-stranded, replicative form DNA molecules is not an absolute requirement for the synthesis of single-stranded DNA, although replicative form DNA accumulation precedes single-strand synthesis in cells infected with wild-type phage. Cells infected at restrictive temperature with the mutant phage do not replicate the infecting DNA, but do accumulate a substantial amount of gene 5 protein, a DNA-binding protein essential for single-strand synthesis. It is proposed that this accumulated gene 5 protein, by binding to the limited number of replicating DNA molecules formed following the shift to the permissive temperature, acts to prevent the synthesis of double-stranded replicative form DNA, thus causing the predominant appearance of single strands. This explanation implies an intermediate common to both single and double-stranded DNA synthesis. The kinetics of gene 5 protein synthesis indicates that it is the ratio of the gene 5 protein to replicating DNA molecules which determines whether an intermediate will synthesize double or single-stranded DNA.