Epigenetics, a term that refers to cellular mechanisms that confer stability of gene expression during development, is a rapidly advancing field of biological and medical research. The main molecular mechanisms that mediate epigenetic phenomena are DNA methylation and chromatin modifications.

Bacteria, as well as eukaryotes, make widespread use of postreplicative DNA methylation for the epigenetic control of DNA-protein interactions. However, bacteria use DNA adenine methylation (rather than DNA cytosine methylation) as an epigenetic signal. DNA adenine methylation plays roles in the virulence of various pathogenic bacteria including Escherichia coli, Salmonella, Vibrio, Yersinia, Haemophilus, and Brucella. In Alphaproteobacteria, methylation of adenine at GANTC sites by the CcrM methylase regulates the cell cycle and couples gene transcription to DNA replication. In Gammaproteobacteria, adenine methylation at GATC sites by the Dam methylase provides signals for DNA replication, chromosome segregation, mismatch repair, packaging of bacteriophage genomes, transposase activity, and regulation of gene expression.

Epigenetic regulation can enable unicellular organisms to respond rapidly to environmental stresses or signals. For example, the yeast prion PSI is generated by a conformational change of the Sup35p translation termination factor, which is then inherited by daughter cells. This can provide a survival advantage under adverse conditions.

Prions can be defined by their ability to catalytically convert other native state versions of the same protein to an infectious conformational state. In this sense they can be viewed as epigenetic agents capable of inducing a phenotypic change without a modification of the genome.

Further reading Epigenetics