Restriction endonucleases and modification methylases in bacteria.

There are three mechanisms by which foreign DNA may enter a prokaryotic organism: conjugation between Gram-negative bacteria, uptake of exogenous DNA (transformation) and infection by viral (bacteriophage) nucleic acid. The interaction between a bacteriophage and its host bacterial cell has been extensively investigated. A bacterium can restrict phage infection through its ability to degrade the infectious DNA on entry into the cell. This is achieved by site-specific deoxyriboendonucleases, called restriction endonucleases, that cleave DNA into a limited, but defined, number of fragments, which are then susceptible to exonuclease attack. The bacterium can prevent cleavage of its chromosomal DNA by site-specific methylation of the nucleotide bases, for which another activity, the DNA-modification methylase, is responsible. The modification methylase can also modify bacteriophage DNA on entry into the cell, thus making the modified phage DNA resistant to the action of the restriction endonuclease. Although the modification methylase protects the host bacterial chromosome, it can also afford protection to the invading phage DNA; as a result, phages grown in a given bacterial strain are insensitive to restriction by that strain.