Question

Are there some examples that can state that how phage may evade the host's restriction system?

Answer 1

Mostly, the bacteria's restriction system against phage consist of R-M immune system i.e. Restriction-Modification system. In this system, two enzymes i.e. Methyl endonuclease (MTase) and Restriction endonulease (REase) provides the bacterial immunity against phage. MTase methylates own genome while REase cut the non-methylated phage genomic material.

However, phages use some specific mechanism to evade host's restriction system.

1. Phages have their own MTases which methylates phage genomic material and they don't get cut by REases of host and thus evade restriction system of host.

2. Various phages have proteins which block the restriction process by host and hence host's restriction system has no effect on them.

3. Most REases require double stranded DNA for restriction. When a phages have single stranded DNA, REases can't cut the foreign DNA.

4. Phages ensure attachment of bulky groups to their genomic material to get rid of restriction by REases of host. Some of the bulky groups are attached by Glycosylation and hydroxymethylation etc.

These are the few examples that explains antirestriction strategies followed by phages to evade restriction systems of hosts.