Restriction of hemimethylated DNA by the Bacillus subtilis R system

The effects of restriction by the BsuR system on hemimethylated SPP1 DNA were investigated. In vitro, single-stranded nicks were introduced in the nonmodified strand of the hemimethylated DNA at the same sites as recognized in nonmodified homoduplex DNA. Transfection with BsuR-treated hemimethylated DNA was severely reduced. In vivo, transfection with hemimethylated DNA was also severely reduced in competent B. subtilis R cells. In contrast, transfection of protoplasts of the R strain with this DNA was not affected. The apparent restriction by competent cells was attributed to the special mode of processing of transfecting DNA.