Question

The unique base in the DNA of the T-even bacteriophages, 5-
hydroxymethylcytosine. The site of glucosylation is shown. What
is the purpose of this glucosylation or base modification event
during the lytic cycle?

Answer 1

Ans.Evolution allowed several bacteriophages to survive in which modified bases which are resistant to many restriction enzymes are incorporated into their genome. One such modification is, modification of cytosines to 5-hydroxymethylcytosine (5hmC) and it can be further modified by α and β glucosylation of the hydroxymethyl group.

Even though 5hmC is resistant to most restriction enzymes, McrA , McrBC and Type IV SauUSI have been shown to specifically restrict its infection in vivo

Primary function of modifications in phage genomes is to prevent cleavage by host restriction endonucleases.

DNA modifications in phage DNA are have other functions also-

• the regulation of operon expression, as demonstrated in the Bacillus phage SPO1 and the coliphage T4
• initiation of DNA packaging into the viral capsid, as shown in coliphage P1.