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Figure 1. The Bertani and Weigle experiment (1953). The discovery of a barrier to infection of 
in its natural host, E.coli K-12, if the phage had been previously propagated on E.coli C. This led to the phenomenon of R-M. Escherichia coli K-12 possesses a R-M system, EcoKI, absent in E.coli C. Phage propagated on E.coli C (called .C or .0) is not protected from restriction by EcoKI and forms plaques with low efficiency (e.o.p.) on E.coli K-12 as compared to E.coli C (bold). Pretreatment of cells with UV leads to RA (italic). The mechanism behind this phenomenon was recently elucidated by Svetlana Makovets and Noreen Murray.
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