Variations in sedimentation patterns among deoxyribonucleic acids synthesized after infection of Escherichia coli by different amber mutants of bacteriophage T7

Abstract

After infection of Escherichia coli by various double-stranded deoxyribonucleic acid (DNA) phages, the newly synthesized phage-directed DNA may sediment differently from the DNA of mature phage particles. There are several reasons why this may occur, in the cases of T4 (7), T5 (13), X (12), and P22 (2), the vegetative phage DNA may be longer than the mature phage DNA, and individual genomes of the phages X (1, 15) and P22 (10) may also be present , intracellularly, as fast-sedimenting circular molecules. Furthermore, it has been shown that some amber mutants of T4 (6) and of X (11), and some temperature-sensitive mutants of P22 (3), determine the synthesis, in nonpermis-sive conditions, of phage-directed DNA whose sedimentation patterns differ from that of the DNA synthesized by the wild-type phages. The study of these patterns is of considerable interest because of the clues it may provide to the steps involved in the synthesis of mature virus genomes. Recently, it has been found (T. J. Kelly, Jr., Federation. Proc., p. 591, 1968) that, during the vegetative growth of phage T7, a replicative intermediate is formed which is a long concate-mer of T7-specific DNA molecules, and whose scission to mature T7 DNA can be inhibited by chloramphenicol. The prevention by this drug of the appearance of a DNA which co-sediments with mature T7 DNA, and the accumulation , under these conditions, of a faster-sedi-menting material containing DNA, had also been observed by one of us (8). Carlson (4) also confirmed Kelly's observation, and she further reported that, after denaturation of the newly synthesized T7 DNA, its sedimentation rate resembles that of mature T7 DNA, which was interpreted as indicating the presence in the long intermediate form of single-stranded nicks about one phage equivalent unit apart. To obtain more information regarding the replica