Two modification methylase genes of Bacillus subtilis R were cloned in Escherichia coli by using a selection procedure which is based on the expression of these genes. Both genes code for DNA-methyl-transferases which render the DNA of the cloning host E. coli HB101 insensitive to the BspRI (5'-GGCC) endonuclease of Bacillus sphaericus R. One of the cloned genes is part of the restriction-modification (RM) system BsuRI of B. subtilis R with specificity for 5'-GGCC. The other one is associated with the lysogenizing phage spβB and produces the methylase M. BsuPβBI with specificity for 5'-GGCC. The fragment carrying the SPβ-derived gene also directs the synthesis in E. coli of a third methylase activity (M. BsuPβBII), which protects the host DNA against HpaII and MspI cleavage within the sequence 5'-CCGG. Indirect evidence suggests that the two SPβB modification activities are encoded by the same gene. No cross-hybridization was detected either between the M. BsuRI and M. BsuPβB genes or between these and the modification methylase gene of B. sphaericus R, which codes for the enzyme M. BspRI with 5'-GGCC specificity.
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