METHYLATED CYTOSINE AT DCM (CC(T)(A)GG) SITES IN ESCHERICHIA-COLI - POSSIBLE FUNCTION AND EVOLUTIONARY IMPLICATIONS

The frequency and distribution of methylated cytosine (5-MeC) at CCA(T )(A)GG (Dcm sites) in 49 E. coli DNA loci (207,530 bp) were determined . Principal observations of this analysis were: (1) Dcm frequency was higher than expected from random occurrence but lower than calculated with Markov chain analysis; (2) CCTGG sites were found more frequently in coding than in noncoding regions, while the opposite was true for CCAGG sites; (3) Dcm site distribution does not exhibit any identifiab ly regular pattern on the chromosome; (4) Dcm sites at oriC are probab ly not important for accurate initiation of DNA replication; (5) 5-MeC in codons was more frequently found in first than in second and third positions; (6) there are probably few genes in which the mutation rat e is determined mainly by DNA methylation. It is proposed that the fun ction of Dcm methylase is to protect chromosomal DNA from restriction- enzyme EcoRII. The Dcm methylation contribution to determine frequency of oligonucleotides, mutation rate, and recombination level, and thus evolution of the E. coli genome, could be interpreted as a consequenc e of the acquisition of this methylation.