THE GENOME OF THE PSEUDO T-EVEN BACTERIOPHAGES, A DIVERGE GROUP THAT RESEMBLES T4

Polymerase chain reaction analysis of a large collection of bacterioph ages with T-even morphology revealed four phages that are distantly re lated to all the others. The genomes of these pseudo T-even phages hyb ridized under stringent conditions to only a limited portion of the T4 genome that encodes virus head, head-to-tail joining and contractile tail genes. Except for this region, no extensive hybridization was det ected between most pairs of the different pseudo T-even genomes. Seque ncing of this conserved region of the pseudo T-even phage RB49 reveale d substantial nucleotide sequence divergence from T4 (similar to 30% t o 40%), and random genomic sequencing of this phage indicated that mor e than a third of its sequences had no detectable homology to T4. Amon g those sequences related to the T-even genes were virion structural c omponents including the constituents of the phage base plate. Only a f ew sequences had homology to T4 early functions; these included ribonu cleotide diphosphatase reductase, DNA ligase and the large subunit of DNA topoisomerase. The genomes of the pseudo T-even phage were digeste d by restriction enzymes that are unable to digest the T-even DNAs whi ch contain glucosylated hydroxymethyl-cytosine residues. This suggests that only Limited nucleotide modifications must be present in the pse udo T-even genomes. Conservation of much of the morphogenetic region o f these diverse phage genomes may reflect particularly strong sequence constraints on these gene products. However, other explanations are c onsidered, including the possibility that the various morphogenetic se gments were acquired by the pseudo T-even genomes by modular evolution . These results support the notion that phage evolution may proceed wi thin a network of both closely and distantly related genomes. (C) 1997 Academic Press Limited.