Question

Strain R of E. coli has an RM system that modifies/cleaves the sequence GAATTC. Sometimes invading phage DNA is methylated by the strain R methylase before the endonuclease can cleave the viral DNA, and then replicates in the methylated form and forms viral particles. If you add some of these viral particles to uninfected bacteria of strain R, what will happen? Select one: a. The invading phage DNA is unmethylated and is protected from the strain Rendonuclease b. The invading phage DNA is methylated and thus cleaved by the strain Rendonuclease c. The invading phage DNA is unmethylated and will be cleaved by the strain Rendonuclease d. The invading phage DNA is methylated and protected from the strain Rendonuclease o e. Strain R is resistant to all phage, no matter the source

Answer 1

Restriction modification (RM) system of the bacteria is a defence system against phage infection. In this system, bacterial genome is methylated by the host methyltransferase enzyme and protects it from its own endonucleases. In such cases, if a phage infectsbthe bacteria gets degraded by the bacterial endonuclease enzyme because phage genome is not methylated.

Suppose if a phage genome gets methylated by the bacterial methyltransferase enzyme, phage genome escapes from the endonuclease enzyme. Subsequently, phage genome gets expressed in the methylated form and viral particles are released from the bacteria.

If these methylated phages again infect the same strain of bacteria, these phages are not get degraded by the endonuclease enzyme because phage genome is already methylated. Hence survive from the RM system of the bacteria.

Hence correct option is d. The invading phage is methylated and protect from the strain R endonuclease.