N6-methyladenine and epigenetic immunity of Deinococcus radiodurans.

DNA methylation is ubiquitously found in all three domains of life. This epigenetic modification on adenine or cytosine residues serves to regulate gene expression or to defend against invading DNA in bacteria. Here, we report the significance of N6-methyladenine (6mA) to epigenetic immunity in Deinococcus radiodurans. Putative protein encoded by DR_2267 ORF (Dam2) contributed 35% of genomic 6mA in D. radiodurans but did not influence gene expression or radiation resistance. Dam2 was characterized to be a functional S-adenosyl methionine (SAM)-dependent N6-adenine DNA methyltransferase (MTase) but with no endonuclease activity. Adenine methylation from Dam2 or Dam1 (N6-adenine MTase encoded by DR_0643) improved DNA uptake during natural transformation. To the contrary, methylation from Escherichia coli N6-adenine MTase (Dam that methylates adenine in GATC sequence) on donor plasmid drastically reduced DNA uptake in D. radiodurans, even in presence of Dam2 or Dam1 methylated adenines. With these results, we conclude that self-type N6-adenine methylation on donor DNA had a protective effect in absence of additional foreign methylation, a separate methylation-dependent Restriction Modification (R-M) system effectively identifies and limits uptake of G6mATC sequence containing donor DNA. This is the first report demonstrating presence of epigenetic immunity in D. radiodurans.