Cloning the KpnI restriction-modification system in Escherichia coli

The genes encoding the KpnI restriction and modification (R-M) system from Klebsiella pneumoniae, recognizing the sequence, 5'-GGTAC decreases C-3', were cloned and expressed in Escherichia coli. Although the restriction endonuclease (ENase)- and methyltransferase (MTase)-encoding genes were closely linked, initial attempts to clone both genes as a single DNA fragment in a plasmid vector resulted in deletions spanning all or part of the gene coding for the ENase. Initial protection of the E. coli host with MTase expressed on a plasmid was required to stabilize a compatible plasmid carrying both the ENase- and the MTase-encoding genes on a single DNA fragment. However, once established, the MTase activity can be supplied in cis to the kpnIR gene, without an extra copy of kpnIM. A chromosomal map was generated localizing the kpnIR and kpnIM genes on 1.7-kb and 3.5-kb fragments, respectively. A final E. coli strain was constructed, AH29, which contained two compatible plasmids: an inducible plasmid carrying the kpnIR gene which amplifies copy number at elevated temperatures and a pBR322 derivative expressing M.KpnI. This strain produces approx. 10 million units of R.KpnI/g of wet-weight cells, which is several 1000-fold higher than the level of R.KpnI produced by K. pneumoniae. In addition, DNA methylated with M.KpnI in vivo does not appear to be restricted by the mcrA, mcrB or mrr systems of E. coli.