Site-specific cleavage of bacteriophage T4 DNA associated with the absence of gene 46 product function

Abstract

A plasmid containing a copy of the late gene 23 was cleaved at two specific locations after bacteriophage T4 infection. Cleavage at the major site, which is at the 3' end of gene 23, was detected only in the absence of gene 46 product function and was independent of the state of modification of cytosine residues. Cutting of plasmid (cytosine-containing) DNA at this site was independent of phage DNA replication and late transcription functions. A second cleavage site, in vector DNA, was also mapped. The minor extent of cutting at this site was independent of gene 46 function. Gene 46 codes for, or controls, an exonuclease involved in T4 DNA recombination and in degradation of cytosine-containing DNA. The coupling of transcription of bacteriophage T4 late genes to replication of the viral DNA remains poorly understood, but some "activation" of the DNA template is thought to be provided by replication (37, 50). We have thought that some progress in defining the nature of DNA competence for late transcription could be made if one examined the effect of T4 infection on the structure of a plasmid containing a T4 late gene. We chose gene 23, which codes for the major capsid protein, for this purpose. Previous experiments have established that the plasmid-borne gene 23 is expressed late in infection and have defined some of the relevant parameters (16, 17). Analysis of deproteinized plasmid DNA uncovered an unexpected effect: a site-specific cleavage of T4 DNA occurred in the absence of gene 46 function. The gene 46 product is involved both in T4 recombination and in degradation of DNA. This report is concerned primarily with the site-specific cleavage. MATERIALS AND METHODS Media. M9S and tryptone broth have been described previously (6). H-2 (low phosphate) medium is medium B-2 of Studier (44), without phosphate, and with 20 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-NaOH (pH 7.5) in place of bis-Tris (G. Kassavetis, personal communicaion.). Bacteria. Escherichia coli CR63 and B40suI were