Bacteriophage T4 gene 17 amplification mutants: evidence for initiation by the T4 terminase subunit gp16.

Bacteriophage T4 genes 16 and 19 containing the 24 bp homology regions that recombine to form Hp17 mutants were cloned into plasmids. When the two homology sequences were cloned either together into one or separately into two compatible plasmids, a polymerase chain reaction assay showed that recombination occurred in vivo. The recombinant sequence was identical with that found in T4 phage Hp17 mutants, and was produced in recombination-deficient Escherichia coli. Mutational analysis revealed a requirement for functional gene 16 but not gene 17 to recombine the sequences. Moreover, gp16, the terminase small subunit, was required, since an amber gene 16 produced the recombinant sequence only when suppressed. Mutations in the gene 16 recombination sequence (3GA and 15TG) that eliminated Hp17 formation in T4 phage increased the synthesis of the large terminase subunit, gp17 in T4 infections, suggesting gp16 interaction with this site. gp16 binding to gene 16 and gene 19 pac-like sites may synapse the homologous sequences to lead to Hp17 mutant formation, and this suggests a synapsis mechanism for control of T4 DNA maturation and concatemer processing in packaging.