Figure 7-1: Integration by site-specific recombination, between the attP site of the phage and the attB site of the host. The two circular molecules form a cointegrate, with junction points attL (left prophage end) and attR (right prophage end). Integration is carried out by a phage-coded protein, integrase. In many phages (including lambda), the reverse reaction (excision) frequently requires a second phage-coded protein, excisonase. Host proteins contributing to the reaction are not shown.