Mutational analysis of the sequence-specific recombination box for amplification of gene 17 of bacteriophage T4

Bacteriophage T4 gene 17 amplification mutants Hp17 that carry two to six tandem repeats of the genes 17-18 region were isolated by growth of gene 17 amber mutants on ochre suppressor strains of Escherichia coli. These mutants arise from an initial sequence-specific recombination between two GCTCA sequences in a 24 bp imperfect homology box in genes 16 and 19. The initial recombination occurred in the wild-type phage T4 population, as shown by polymerase chain reaction, at a frequency of about 10(-6), which is consistent with the frequency of mutant isolation. T4 phage with mutations of the 3rd, 6th, 9th, 12th, or 15th positions in the 24 bp box of gene 16 either failed to produce gene amplification mutant Hp17 or produced gene amplification mutants from an initial recombination at other regions. Among the mutants that failed to produce gene amplification mutants, the initial recombination generally occurred at lower frequencies at either the GCTCA sequence or other sequences. Since the gene amplification mutations are eliminated or shifted to different sequences by base changes that increase as well as decrease homology, the predominant recombination event between the gene 16 and 19 recombination boxes appears to be sequence-dependent rather than homology-dependent.