THE ECO72IC GENE SPECIFIES A TRANS-ACTING FACTOR WHICH INFLUENCES EXPRESSION OF BOTH DNA METHYLTRANSFERASE AND ENDONUCLEASE FROM THE ECO72IRESTRICTION-MODIFICATION SYSTEM

Eco72I from Escherichia coli RFL72 is a type-II restriction-modificati on (R-M) system recognizing and cleaving the sequence 5'-CAC down arro w GTG-3'. The R-M genes are transcribed divergently and between the tw o genes is a small open reading frame codirectional to the R gene. Thi s small ORF acts both to stimulate ENase expression and to depress DNA methyltransferase synthesis. The activity of beta Gal produced from t he eco721M::lacZ translational fusion increased tenfold, and eco72IR:: lacZ translational fusion beta Gal activity decreased 130-fold when ec o72IC was inactivated by a frameshift mutation. Analysis of nucleotide sequences of R-M systems, containing C genes, revealed a 5'-ACCTTATAG TC-3' consensus sequence upstream from the regulatory genes in all six analysed R-M systems. This sequence, named C-box, may play the role o f an operator sequence.