Comparative sequence analysis of the DNA packaging, head, and tail morphogenesis modules in the temperate cos-site Streptococcus thermophilus bacteriophage Sfi21.
The temperate Streptococcus thermophilus bacteriophage Sfi21 possesses 15-nucleotide-long cohesive ends with a 3' overhang that reconstitutes a cos-site with twofold hyphenated rotational symmetry. Over the DNA packaging, head and tail morphogenesis modules, the Sfi21 sequence predicts a gene map that is strikingly similar to that of lambdoid coliphages in the absence of any sequence similarity. A nearly one to one gene correlation was found with the phage lambda genes Nu1 to H, except for gene B-to-E complex, where the Sfi21 map resembled that of coliphage HK97. The similarity between Sfi21 and HK97 was striking: both major head proteins showed an N-terminal coiled-coil structure, the mature major head proteins started at amino acid positions 105 and 104, respectively, and both major head genes were preceded by genes encoding a possible protease and portal protein. The purported Sfi21 protease is the first viral member of the ClpP protease family. The prediction of Sfi21 gene functions by reference to the gene map of intensively investigated coliphages was experimentally confirmed for the major head and tail gene. Phage Sfi21 shows nucleotide sequence similarity with Lactococcus phage BK5-T and a lactococcal prophage and amino acid sequence similarity with the Lactobacillus phage A2 and the Staphylococcus phage PVL. PVL is a missing link that connects the portal proteins from Sfi21 and HK97 with respect to sequence similarity. These observations and database searches, which demonstrate sequence similarity between proteins of phage from gram-positive bacteria, proteobacteria, and Archaea, constrain models of phage evolution.