Structural organization of the ori site of phage P22; comparison with other lambdoid ori sites

The homologous DNA regions of phages P22, lambda and lambdoid coliphages, which code for the amino-terminal portion of genes 18 or O, contain the ori signal. Both the lambdoid and P22 ori regions can be divided into sections, A, B and C. The four direct repeats with internal rotational symmetry of section A in P22 are less regularly organized than in the corresponding signals of the phi 80 and lambda ori sites and show greatest homology to coliphage phi 82. Section B is rich in adenines in the l strand, and section C can be recognized in the P22 ori by the occurrence of overlapping inverted repeats. The latter region is not homologous to the structurally similar section C, 'EcoRI-loop', of the lambdoid coliphages. The results further define the specificity determinants of lambdoid O protein-ori interactions and demonstrate the evolutionary relationship between these functional units.