C. Monod et al., THE GENOME OF THE PSEUDO T-EVEN BACTERIOPHAGES, A DIVERGE GROUP THAT RESEMBLES T4, Journal of Molecular Biology, 267(2), 1997, pp. 237-249
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.