Bacteriophage P22 binds to its cell surface receptor, the repetitive O-antigen structure in Salmonella lipopolysaccharide, by its six homotrimeric tailspikes. Receptor binding by soluble tailspikes and the receptor-inactivating endorhamnosidase activity of the tailspike protein were studied using octa- and dodecasaccharides comprising two and three O-antigen repeats of Salmonella enteritidis and Salmonella typhimurium lipopolysaccharides. Wild-type tailspike protein and three mutants (D392N, D395N, and E359Q) with defective endorhamnosidase activity were used. Oligosaccharide binding to all three subunits, measured by a tryptophan fluorescence quench or by fluorescence depolarization of a coumarin label attached to the reducing end of the dodecasaccharide, occurs independently. At 10 degrees C, the binding affinities of all four proteins to oligosaccharides from both bacterial strains are identical within experimental error, and the binding constants for octa- and dodecasaccharides are 1 x 10(6) M(-1) and 2 x 10(6) M(-1), proving that two O-antigen repeats are sufficient for lipopolysaccharide recognition by the tailspike. Equilibration with the oligosaccharides occurs rapidly, but the endorhamnosidase produces only one cleavage every 100 s at 10 degrees C or about 2 min(-1) at the bacterial growth temperature. Thus, movement of virions in the lipopolysaccharide layer before DNA injection may involve the release and rebinding of individual tailspikes rather than hydrolysis of the O-antigen.