Enzymic and Molecular Properties of Base‐Plate Parts of Bacteriophage P22

Using 14 C‐labeled Salmonella bacterial cells as the substrate, the enzymic and molecular properties of the base‐plate parts of phage P22 were studied. The base‐plate part consisted of a single protein species which cleaved extensively the O‐antigen of Salmonella typhimurium, Salmonelly schotimuellerie and with somewhat slower rate that of Salmonella typhi , releasing oligosaccharide products with rhamnose at the reducing end. Much less cleavage was observed with a strain of S. typhimurium lysogenic for P22, and no significant reaction with Salmonella anatum, Salmonella newington and Salmonella minneapolis . The base‐plate part enzyme was a very heat‐stable protein and only 10–20% loss was observed after treatment at 85 °C for 5 min. The pH optimum of the enzyme was around 7.5, and the glycosidase activity was not influenced by the ionic strength (25–250 mM) of the medium or the presence of Mg 2+ . The molecular weight of the base‐plate part was 320000 by sedimentation equilibrium. Dodecylsulphate‐acrylamide gel electrophoresis revealed a single band of molecular weight 77000, indicating that a single base‐plate part corresponds to a tetramer of identical subunits. Circular dichroism spectra of P22 base‐plate parts showed a major contribution of β structure. The protein was rich in acidic amino acids, glycine and serine.