The adsorption protein of phage IKe. Localization by deletion mutagenesis of domains involved in infectivity.

We constructed a set of plasmid-encoded internal deletion mutants within the gene for the adsorption protein (g3p) of phage IKe. All mutant proteins still contain the signal and membrane anchor sequence, as those are known to be indispensable for proper localization and hence assembly of the g3p into phage. These various deletions comprise all internal parts of the protein and are properly incorporated into phage, which remarkably shows that signal and anchor sequence are sufficient for incorporation of g3p. The data furthermore reveal that two separate sections within the IKe g3p are essential for infection: one amino-terminal, preceding the glycine-rich stretch, and the other carboxy-terminal. We conclude that this latter domain is involved in penetration because mutants lacking it are not infectious, but still bind to the receptor. The amino-terminal region, essential for infection, bears the receptor-recognizing domain and a sequence homologous to the penetration domain of the evolutionary related Ff phages, which is probably also involved in penetration of phage IKe. The prominent glycine-rich stretch of the IKe g3p is not essential for infection but significantly promotes it.