The DNA region of phage BF23 encoding receptor binding protein and receptor blocking lipoprotein lacks homology to the corresponding region of closely related phage T5.

Analysis of the DNA region upstream of the BF23 hrs gene revealed a genetic organisation similar to that of closely related phage T5. A gene encoding a lipoprotein (llp(BF23)) is located directly upstream of the gene encoding the receptor binding protein (hrs) but is transcribed in opposite direction. The gene is followed by four open reading frames transcribed in the same direction. The llp (BF23) gene product does not show similarity to the corresponding T5 Llp(T5) protein, however, like Llp(T5) does for FhuA it blocks the BtuB receptor for BF23 infection. While no similarity between BF23 and T5 was observed for the DNA region encoding Llp and the receptor binding protein, the flanking regions were highly similar. Based on our results we conclude that a genetic module, the receptor-binding/receptor-blocking module, exists in phages BF23 and T5. Due to exclusion of homologous recombination within this module, it is hereditary only as an intact module: separation of the receptor-binding gene from the receptor-blocking gene, which apparently results in reduced fitness of the phage due to inactivation of progeny phage by active receptor proteins in the outer membranes of lysed cells, is thus effectively prevented.