Degrees of relatedness of T‐even type E. coli phages using different or the same receptors and topology of serologically cross‐reacting sites.

The relatedness of a series of T‐even like phages which use the Escherichia coli outer membrane protein OmpA as a receptor, and the classical phages T2, T4 and T6 has been investigated. Immunoelectron microscopy and the pattern of phage resistance in bacterial mutants revealed that: (i) phages of this morphology do not necessarily cross‐react serologically; (ii) phages using different receptors may bind heterologous IgG everywhere except to the tip (comprising approximately 10% of one fiber polypeptide) of the long tail fibers; (iii) cross‐reacting OmpA‐specific phages may bind heterologous IgG only to the tip of these fibers: (iv) OmpA‐specific phages not cross‐reacting at the tip of the tail fibers use different receptor sites on the protein. Absence of cross‐reactivity appears to reflect high degrees of dissimilarity. A DNA probe consisting of genes encoding the two most distal tail fiber proteins of T4 detected homologies only in DNA from phages serologically cross‐reacting at this fiber. Even under conditions of low stringency, allowing the formation of stable hybrids with almost 30% base mismatch, no such homologies could be found in serologically unrelated phages. Thus, in the collection of phages examined, there are sets of very similar and very dissimilar tail fiber genes and even of such gene segments.