A novel mutant of bacteriophage T7 that is defective in early phage DNA synthesis.

A mutant of bacteriophage T7 is described which produces smaller plaques than the wild type and is defective in early phage DNA synthesis. The mutation is located in the Class II transcriptional region of the T7 genome together with all the other genes involved in phage DNA synthesis, but it could not be placed into any of the existing known T7 genes. DNA replication in strain R9 begins at the same time as for wild type although it proceeds very slowly until 15 minutes after infection, after which time DNA synthesis is apparently normal. It is concluded therefore that there are two types of DNA replication in phage T7, which differ with respect to their dependence on the mutant function. The change from one mode to the other is marked by the formation of folded, complex DNA inside the cell.