Mutant of Escherichia coli which blocks T7 bacteriophage assembly: Accumulation of short T7 DNA

A mutant strain which blocks bacteriophage T7 assembly, presumably at the phage DNA packaging process, was isolated from Escherichia coli D10 F −. This mutant, Y49, was selected for its ability to plate T7 + phage but not T7 gene 0.7 mutant phage (T7K −). The plating efficiency of T7 + phage on strain Y49 was over 50% of that on the parental strain but the yield of T7 + phage was less than 10%. Upon infection with T7 + or T7K − phage, Y49 cells synthesized T7-specific RNA and protein almost normally but the synthesis of T7 progeny DNA was slightly reduced, about 70 to 80% of the amount synthesized by the parental strain. In Y49 cells infected with T7 + or T7K − phage, T7 progeny DNA appeared first as concatemers larger in size than the mature T7 phage DNA but it later was converted to pieces about 30%, shorter than the size of T7 phago DNA. When Y49 cells were infected with T7 mutant phages known to accumulate concatemeric T7 progeny DNA in the normal host cells, T7 progeny DNA also accumulated as concatemers and was not cleaved to smaller pieces. These results suggest that the cleavage of concatemeric T7 DNA in T7 + or T7K − phage-infected Y49 cells, which generates short DNA species, appears to be site-specific and probably results from a blockage of the phage DNA packaging process due to the host mutation, implying that a host function is involved in the T7 phage DNA packaging process.