T-even bacteriophages (T2, T4, T6) contain DNA with modified cytosine residues. Two examples can be seen here. Furthermore, most of the 5-hydroxymethylcytosine residues in these viruses are in the form of -glucosyl-5-hydroxymethylcytosine (i.e., their hydroxymethyl groups have formed a glycosidic linkage with glucose moieties). The enzymes responsible for these modifications are encoded by the virus. Figure 22.23 shows the pathway to formation of glucosylated DNA from T-even phage salvage enzymes. The viruses also contain DNAses specific for DNA lacking the cytosine modifications (such as host DNA), thus providing the virus with a ready supply of dCMP for the salvage synthesis shown in Figure 22.23.

Other modifications made by viruses include the following:

1. Some Bacillus subtilis phages substitute uracil for thymine in their DNA

2. Some Bacillus subtilis phages contain 5-hydroxymethyluracil in place of thymine.

3. A phage of Xanthomonas oryzae substitutes 5-methylcytosine for every one of the cytosines in its DNA .

In each case, the modifying enzymes necessary to make the changes above are encoded by the virus.

1. All the Virology on the WWW

2. Bacteriophage T4 World-Wide Home