Effect of Bacillus subtilis BsuM restriction-modification on plasmid transfer by polyethylene glycol-induced protoplast fusion

Polyethylene glycol (PEG)-induced cell fusion is a promising method to transfer larger DNA from one cell to another than conventional genetic DNA transfer systems. The laboratory strain Bacillus subtilis 168 contains a restriction (R) and modification (M) system, BsuM, which recognizes the sequence 5'-CTCGAG-3'. To study whether the BsuM system affects DNA transfer by the PEG-induced cell fusion between R ⁺M ⁺ and R ^-M ^- strains, we examined transfer of plasmids pHV33 and pLS32neo carrying no and eight BsuM sites, respectively. It was shown that although the transfer of pLS32neo but not pHV33 from the R ^-M ^- to R ⁺M ⁺ cells was severely restricted, significant levels of transfer of both plasmids from the R ⁺M ⁺ to R ^-M ^- cells were observed. The latter result shows that the chromosomal DNA in the R ^-M ^- cell used as the recipient partially survived restriction from the donor R ⁺M ⁺ cell, indicating that the BsuM R ^-M ^- strain is useful as a host for accepting DNA from cells carrying a restriction system(s). Two such examples were manifested for plasmid transfer from Bacillus circulans and Bacillus stearothermophilus strains to a BsuM-deficient mutant, B. subtilis RM125.