Practice question

Bacteriophage lambda integration into Escherichia coli chromosome is classic tyrosine recombinase system establishing lysogeny. Integrase Int recognizes attachment sites attP on circular phage genome and attB located within bacterial gal/bio region. Both sites contain common 15 base-pair core O where crossovers occur, flanked by arm-type binding sites for Int, IHF, and auxiliary factors. Int tetramer catalyzes reciprocal exchange between attP and attB, integrating prophage flanked by hybrid attL and attR sites, reaction favored by Integration Host Factor induced DNA bending and supercoiling in absence of excisionase.