Site-specific recombination of bacteriophage P22 does not require integration host factor

Abstract

Site-specific recombination by phages lambda and P22 is carried out by multiprotein-DNA complexes. Integration host factor (IHF) facilitates lambda site-specific recombination by inducing DNA bends necessary to form an active recombinogenic complex. Mutants lacking IHF are over 1,000-fold less proficient in supporting lambda site-specific recombination than wild-type cells. Although the attP region of P22 contains strong IHF binding sites, in vivo measurements of integration and excision frequencies showed that infecting P22 phages can perform site-specific recombination to its maximum efficiency in the absence of IHF. In addition, a plasmid integration assay showed that integrative recombination occurs equally well in wild-type and ihfA mutant cells. P22 integrative recombination is also efficient in Escherichia coli in the absence of functional IHF. These results suggest that nucleoprotein structures proficient for recombination can form in the absence of IHF or that another factor(s) can substitute for IHF in the formation of complexes.

FIG. 1

PCR analysis to detect P22 prophage integrated into the ataA site of S. typhimurium. PCR products were analyzed by electrophoresis on a 1.5% agarose gel. The nucleotide sequences of primers are shown in Table 2. Primer sets P+ and B−, P− and B+, B+ and B, and P+ and P− were used to amplify the 918-bp attL-containing fragments, 362-bp attR-containing fragments, 246-bp ataA-containing fragments, and 1,034-bp attP-containing fragments, respectively (Table 2). DNA templates were purified from MS1868 (lanes 2, 5, 8, and 11), MS1868(P22) lysogen (lanes 3, 6, 9, and 12), JG1151(P22) lysogen (lanes 4, 7, 10, and 13), and P22 (lane 14). Lane 1 contains molecular size markers, which are φX174 DNA digested with restriction enzyme HaeIII. Samples were amplified with primers P+ and B− (lanes 2 to 4), P− and B+ (lanes 5 to 7), B+ and B− (lanes 8 to 10), and P+ and P− (lanes 11 to 14).