The XmnI restriction-modification system: Cloning, expression, sequence organization and similarity between the R and M genes

The xmnIRM genes from Xanthomonas manihotis 7AS1 have been cloned and expressed in Escherichia coli. The nucleotide (nt) sequences of both genes were determined. The XmnI methyltransferase (MTase)-encoding gene is 1861 bp in length and codes for 620 amino acids (aa) (68 660 Da). The restriction endonuclease (ENase)-encoding gene is 959 bp long and therefore codes for a 319-aa protein (35 275 Da). The two genes are aligned tail to tail and they overlap at their respective stop codons. About 4 x 10(4) units/g wet cell paste of R . XmnI was obtained following IPTG induction in a suitable E. coli host. The xmnIR gene is expressed from the T7 promoter. M . XmnI probably modifies the first A in the sequence, GAA(N)(4)TTC, The xmnIR and M genes contain regions of conserved similarity and probably evolved from a common ancestor, M . XmnI is loosely related to M . EcoRI. The XmnI R-M system and the type-I R-M systems probably derived from a common ancestor.