Physical evidence for early transcription in intracellular bacteriophage P2 DNA

Abstract

Escherichia coli were infected with bacteriophage P2, and the phage DNA replication was blocked by a mutation in the phage A gene. The resulting intracellular DNA molecules contained displacement loops, which were shown to contain RNA. The positions of these R loops were mapped by electron microscope partial denaturation mapping, and 90% were found to be located in the same region on the DNA, spanning less than 15% of the P2 chromosome. Also, most of the R loops have a common left end located at 77 ± 1.4% on the P2 DNA map, which corresponds well with the position of the promoter for the P2 early genes as determined by the position of several P2 virulent deletion mutations. We conclude that the observed R loops on intracellular P2 DNA represent transcription from the promoter for the early operon in P2.