The relationship between HP1 and S2 bacteriophages of Haemophilus influenzae

Comparison of the nucleotide sequences of the left arms of two Haemophilus influenzae phages, S2 and HP1 is presented. They exhibit a characteristic mosaic pattern of homologous and non-homologous regions. The homology extends over the attP site and int, orf 5 to 9, rep and the 3' part of cI genes. Two major non-homologous regions were detected. One is found between the int and cI genes; the other spans the region of promoters and the cox gene. Variations in the region of the promotors which is involved in the choice between a lysogenic and a lytic pathway and some divergences in the cI coding sequences are probably responsible for the observed immunity differences between the two phages. Distinctions in the distribution of consensus sequences for an integration host factor (IHF) and integrase-binding sites and promoters are described. These data offer an explanation of the relationship between three types of S2/HP1 phages. It allows in turn a final settlement of the nomenclature variation in the literature. The results presented, which are similar to those obtained for other phage groups, suggest that the mosaic structure of phage genomes is a normal outcome of phage divergence.