Defective DNA injection by alkylated and nonalkylated bacteriophage T7

DNA injection by alkylated and nonalkylated bacteriophage T7 has been analyzed by a physical method which involved Southern hybridization to identify noninjected regions of DNA. Treatment of phage with methyl methanesulfonate reduced the amount of DNA injected into wild-type Escherichia coli cells. This reduction was correlated with a decreased injection of DNA segments located on the right-hand third of the T7 genome. An essentially identical injection defect was observed when alkylated phage infected E. coli mutant cells unable to repair 3-methyladenine. Furthermore, untreated phage particles were discovered to be naturally injection-defective. Some injected all their DNA except those segments located in the rightmost 15% of the T7 genome, while other injected no DNA at all. In the presence of rifampicin, untreated phages injected only segments from the left end of the genome. These results provide direct physical evidence that T7 DNA injection is strictly unidirectional, starting from the left end of the T7 genome. The injection defect quantified here for alkylated phage is probably partially, if not totally, responsible for phage inactivation, when that inactivation is measured in wild-type E. coli cells. Since alkylated phage injected the same DNA sequences into both wild-type and repair-deficient cells, we conclude that DNA injection is independent of the host-cell's capacity for repair of 3-methyladenine residues.