Characterization of the type IV restriction modification system BspLU11III from Bacillus sp. LU11

We report the characterization and cloning of the genes for an unusual type IV restriction–modification system, BspLU11III, from Bacillus sp. LU11. The system consists of two methyltransferases and one endonuclease, which also possesses methyltransferase activity. The three genes of the restriction–modification system, bsplu11IIIMa, bsplu11IIIMb and bsplu11IIIR, are closely linked and tandemly arranged. The corresponding enzymes recognize the dsDNA sequence 5′-GGGAC-3′/5′-GTCCC-3′, with M.BspLU11IIIa modifying the A (underlined) of one strand and M.BspLU11IIIb the inner C (underlined) of the other strand. R.BspLU11III has both endonuclease and adenine-specific methyltransferase activities and is able to protect the DNA against cleavage by itself. In contrast to all type IV restriction–modification systems described so far, which have only one adenine-specific methyltransferase, BspLU11III is the first type IV restriction–modification system that includes two methyltransferases, one of them being cytosine specific.