MUTATIONAL ANALYSIS OF PROTEIN-BINDING SITES INVOLVED IN FORMATION OFTHE BACTERIOPHAGE-LAMBDA ATTL COMPLEX

Bacteriophage lambda site-specific recombination requires the formatio n of higher-order protein-DNA complexes to accomplish synapsis of the partner attachment (att) sites as well as for the regulation of the in tegration and excision reactions, The aff sites are composed of a core region, the actual site of strand exchange, and flanking arm regions, The attL site consists of two core sites (C and C'), an integration h ost factor (IHF) binding site (H'), and three contiguous Int binding a rm sites (P'1, P'2, and P'3), In this study, we employed bacteriophage P22 challenge phages to determine which protein binding sites partici pate in attL, complex formation in vivo, The C', H', and P'1 sites wer e critical, because mutations in these sites severely disrupted format ion of the attL complex, Mutations in the C and P'2 sites were less se vere, and alteration of the P'3 site had no effect on complex formatio n, These results support a model in which IHF, bound to the H' site, b ends the attL DNA so that the Int molecule bound to pll also interacts with the C' core site, This bridged complex, along with a second Int molecule bound to P'2, helps to stabilize the interaction of a third I nt with the C core site, The results also indicate that nonspecific DN A binding is a significant component of the Int-core interactions and that the cooperativity of Int binding can overcome the effects of muta tions in the individual arm sites and core sites.