The structural gene module in Streptococcus thermophilus bacteriophage phi Sfi11 shows a hierarchy of relatedness to Siphoviridae from a wide range of bacterial hosts.

The structural gene cluster and the lysis module from lytic group II Streptococcus thermophilus bacteriophage phi Sfi11 was compared to the corresponding region from other Siphoviridae. The analysis revealed a hierarchy of relatedness. phi Sfi11 differed from the temperate S. thermophilus bacteriophage phi O1205 by about 10% at the nucleotide level. The majority of the changes were point mutations, mainly at the third base position. Only a single gene (orf 695) differed substantially between the two phages. Over the putative minor tail and lysis genes, phi Sfi11 and the lytic group 1 S. thermophilus phi Sfi19 shared regions with variable degrees of similarity. Orf 1291 from phi Sfi19 was replaced by four genes in phi Sfi11, two of which (orf 1000 and orf 695) showed a complicated pattern of similarity and nonsimilarity compared with phi Sfi19. The predicted orf 695 gp resembles the receptor-recognizing protein of T-even coliphages in its organization, but not its sequence. No sequence similarity was detected between phi Sfi11 and phi Sfi19 in the region covering the major head and tail genes. Comparison of the structural gene map of phi Sfi11 with that of Siphoviridae from gram-positive and -negative bacterial hosts revealed a common genomic organization. Sequence similarity was only found between phi Sfi11 and Siphoviridae from gram-positive hosts and correlated with the evolutionary distance between the bacterial hosts. Our data are compatible with the hypothesis that the structural gene operon from Siphoviridae of the low G + C group of gram-positive bacteria is derived from a common ancestor.