A comparative study of Shigella sonnei phages U and G and Escherichia coli phage T4 has shown that enzymes coded for by the Sh. sonnei phages can functionally substitute for some T4-coded products. This finding in indicative of an evolutionary relationship between T-even phages and disenteric phages U and G. The U phage is uncapable to compensate amber mutants for the genes that control the conversion of cytosine into 5-hydroxymethyl cytosine (5-HMC) and the glucosylation of the latter, which agrees with our earlier finding that the U phage DNA contains no 5-HMC. U and G phages are also found to exclude the T4 phage in the course of mixed infection.
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