The genes of the 5′-GGCC specific BsuRI restriction-modification system of Bacillus subtills have been cloned and expressed in E. coll and their nucleotide sequence has been determined. The restriction and modification genes code for polypeptides with calculated molecular weights of 66,314 and 49,642, respectively. Both enzymes are coded by the same DNA strand. The restriction gene is upstream of the methylase gene and the coding regions are separated by 780 bp. Analysis of the RNA transcripts by S1-nuclease mapping indicates that the restriction and modification genes are transcribed from different promoters. Comparison of the amino acid sequences revealed no homology between the BsuRI restriction and modification enzymes. There are, however, regions of homology between the BspRI SPR methylase and two other GGCC specific modification enzymes, the BspRI and SPR methylases.
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