THE XMNI RESTRICTION-MODIFICATION SYSTEM - CLONING, EXPRESSION, SEQUENCE ORGANIZATION AND SIMILARITY BETWEEN THE R-GENE AND M-GENE

The xmnIRM genes from Xanthomonas manihotis 7AS1 have been cloned and expressed in Escherichia coli. The nucleotide (nt) sequences of both g enes 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 al igned tail to tail and they overlap at their respective stop codons. A bout 4 x 10(4) units/g wet cell paste of R . XmnI was obtained followi ng IPTG induction in a suitable E. coli host. The xmnIR gene is expres sed from the T7 promoter. M . XmnI probably modifies the first A in th e sequence, GAA(N)(4)TTC, The xmnIR and M genes contain regions of con served similarity and probably evolved from a common ancestor, M . Xmn I is loosely related to M . EcoRI. The XmnI R-M system and the type-I R-M systems probably derived from a common ancestor.