A novel terminase activity associated with the DNA packaging protein gp17 of bacteriophage T4

The mechanism of generation of circularly permuted ends in bacteriophage T4 by a strictly headful packaging process has remained unresolved since its proposal by Streisinger et al. (1967, Proc. Natl. Acad. Sci. USA 57, 292-295). In this paper, we show that the phage T4 DNA packaging proteins gp16 and gp17 act as T4 terminase. Expression of gp16 and gp17 in Escherichia coli resulted in extensive cleavage of both plasmid as well as E. coli genomic DNAs. Analysis of a number of recombinant terminase constructs, and mutants in gene 17, demonstrated that the active site for cleavage of DNA is located in gp17, but not in gp16. Consistent with the circularly permuted nature of phage T4 ends, cleavage by gp17 occurred in a sequence independent manner generating random ends. The terminase cutting occurred preferentially on a DNA substrate that is transcriptionally active. We propose that a structural feature in the transcriptionally active region provides a site for attachment of terminase to DNA following which the terminase moves along the DNA, and cleaves at a random sequence.