E. coliK-12 contains DNA with a specific pattern of methylation.

All cells infected by phage and phage replicates.

Cells lyse, releasing phage. These phage can infect K-12 but rarely B.

Figure 13.13 Restriction-Modification System The ability of phage to infect and replicate in a strain of E. coli other than the one it last infected is limited by the restriction-modification system.

enzyme of B. Likewise, the methylation of the bases in DNA by the modification enzyme of K-12, which protects the DNA from degradation in K-12, does not protect it from degradation in B. This explains why a strain always has a combination of the two genes, one which codes for a restriction enzyme and the other which codes for a corresponding methylating enzyme.

The reason that phage released from only a few B cells can then infect other B cells at a high efficiency can also be explained by the restriction-modification system. In most cases, foreign DNA entering a cell is degraded by the restriction enzyme before it has a chance to go through its replication cycle. In rare cases, however, the entering foreign DNA is methylated by the modification enzyme of the strain before the restriction enzyme has the chance to degrade it. Once the specific bases are methylated, the foreign DNA is converted by the cell to appear non-foreign. This DNA will replicate, and phage will be released that can now infect and lyse the same host strain at a high efficiency.

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