Arm site independence of coliphage HK022 integrase in human cells

The integrase encoded by the lambdoid phage HK022 (Int-HK022) resembles its coliphage counterpart (Int-lambda) in the roles of the cognate DNA arm binding sites and in controlling the direction of the reaction. We show here that within mammalian cells, Int-HK022 does not exhibit such a control. Rather, Int-HK022 recombined between all ten possible pairwise att site combinations, including attB x attB that was more effective than the conventional integrative attP x attB reaction. We further show that Int-HK022 depends on the accessory integration host factor (IHF) protein considerably less than Int-lambda and exhibits stronger binding affinity to the att core. These differences explain why wild-type Int-HK022 is active in mammalian cells whereas Int-lambda is active there only as an IHF-independent mutant.