Cloning and characterization of the BglII restriction-modification system reveals a possible evolutionary footprint

Bg/II, a type II restriction-modification (R-M) system from Bacillus globigii, recognizes the sequence 5'-AGATCT-3'. The system has been cloned into E. coli in multiple steps: first the methyltransferase (MTase) gene, bg/IIM, was cloned from B. globigii RUB561, a variant containing an inactivated endonuclease (ENase) gene (bg/IIR). Next the ENase protein (R.Bg/lI) was purified to homogeneity from RUB562, a strain expressing the complete R-M system. Oligonucleotide probes specific for the 5' end of the gene were then synthesized and used to locate bg/IIR, and the gene was isolated and cloned in a subsequent step. The nucleotide sequence of the system has been determined, and several interesting features have been found. The genes are tandemly arranged, with bg/IIR preceding bg/IIM. The amino acid sequence of M.Bg/II is compared to those of other known MTases. A third gene encoding a protein with sequence similarity to known C elements of other R-M systems is found upstream of bg/IIR. This is the first instance of a C gene being associated with an R-M system where the R and M genes are collinear. In addition, open reading frames (ORFs) resembling genes involved with DNA mobility are found in close association with Bg/II. These may shed light on the evolution of the R-M system.