Amber Mutants of Bacteriophages T3 and T7 Defective in Phage-directed Deoxyribonucleic Acid Synthesis

Abstract

Amber mutants of the related phages T3 and T7 were isolated and tested for their ability to restore—as the wild type does—thymidine incorporation in ultraviolet (UV)-irradiated, UV-sensitive, nonpermissive host bacteria ( Escherichia coli B s-1 ). Most amber mutants had this ability. However, in both T3 and T7, mutants unable to promote thymidine incorporation under these conditions were found and classified into two well-defined complementation groups: T3DO-A and T3DO-B, T7DO-A and T7DO-B. Infection of B s-1 cells with representatives of groups DO-A had the following characteristics: (i) phage-directed uridine uptake in UV-irradiated cells was reduced to less than 20% of normal; (ii) breakdown of host deoxyribonucleic acid (DNA) was delayed and incomplete; (iii) no serum-blocking antigens appeared; (iv) no cell lysis occurred; (v) the ability to exclude the heterologous wild type was impaired. Amber mutants of the DO-B groups, infecting B s-1 , were able to: (i) promote an efficient phage-directed uridine uptake in UV-irradiated cells; (ii) bring about rapid breakdown of host DNA; (iii) synthesize serum-blocking antigens; (iv) lyse the host cells, generally after the normal latent period; (v) exclude efficiently the heterologous wild type. Although physiological similarities between the respective DO-A mutants or DO-B mutants of T3 and T7 were evident, no physiological cross-complementation occurred, and genetic crosses gave no evidence of genetic homologies between groups of T3 and T7.