CLONING AND SEQUENCES OF THE GENES ENCODING THE CFRBI RESTRICTION-MODIFICATION SYSTEM FROM CITROBACTER-FREUNDII

The genes encoding the CfrBI restriction and modification (R-M) system s from Citrobacter freundii and recognizing the sequence 5'-CCWWGG-3' (W = A or T) were cloned in Escherichia coli McrBC- cells. The nucleot ide (nt) sequences of the genes were determined. Two large open readin g frames were found. Deletion analysis showed that one of them 1128 n t coding for 376 amino acids (aa)! corresponds to a methyltransferase (MTase)-encoding gene and the other (1065 nt coding for 355 aa) to a r estriction endonuclease-encoding gene. The genes are oriented divergen tly and separated by 76 bp. A CfrBI site (5'-m4CCATGG) was found in th e intergenic region of the cfrBIRM genes. Analysis of the deduced aa s equence of M.CfrBI made it possible to determine the typical features of a m4C-specific MTase. Limited homology between the M.CfrBI and R.Cf rBI proteins was also found.