THE NIAIV RESTRICTION AND MODIFICATION GENES OF NEISSERIA-LACTAMICA ARE FLANKED BY LEUCINE BIOSYNTHESIS GENES

The genes encoding the Neisseria lactamica restriction endonuclease IV (R.NIaIV) and its cognate DNA methyltransferase (M.NlaIV), both of wh ich recognize the sequence GGNNCC, have been cloned in Escherichia col i and overexpressed using the T7 polymerase/promoter system. Analysis of a sequenced 3.58kb fragment established the gene order, leuD-M.NlaI V-R.NlaIV-leuB. The predicted primary sequence of M.NlaIV (423 amino a cids) shows the highest degree of identity to a pair of cytosine-speci fic methyltransferases, M.BanI (44.9%) and M.HgiCI (44.3%), which reco gnize the sequence GGYRCC (Y, pyrimidines; R, purines). In contrast, t he R.NlaIV protein sequence (243 amino acids) is unique in the existin g database, a situation that holds for most endonucleases. Flanking th e NlaIV modification and restriction genes are homologues of the leuD and leuB genes of enteric bacteria, which code for enzymes in the leuc ine biosynthesis pathway. This gene context implies a possible new mod e of gene regulation for the RM.NlaIV system, which would involve a me chanism similar to the recently discovered leucine/Lrp regulon in E. c oli.