Mc. Gomezeichelmann et J. Ramirezsantos, METHYLATED CYTOSINE AT DCM (CC(T)(A)GG) SITES IN ESCHERICHIA-COLI - POSSIBLE FUNCTION AND EVOLUTIONARY IMPLICATIONS, Journal of molecular evolution, 37(1), 1993, pp. 11-24
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.