For several decades, researchers have known that it is possible to modify synthetic oligonucleotides (short strands of DNA) by adding sulfur to the sugar-phosphate DNA backbone as a phosphorothioate. Researchers often use such modifications in the laboratory to make DNA resistant to nucleases (enzymes that snip DNA in certain locations) as a step toward gene and antisense therapies of human diseases.

Dedon said he and his co-workers were surprised to discover that a group of bacterial genes, known as the dnd gene cluster, gives bacteria the ability to employ the same modification on their own. “It turns out that nature has been using phosphorothioate modifications of DNA all along, and we just didn’t know about it,” he said.
…
He theorizes that the modification system might serve as either protection against foreign (unmodified) DNA, or as a “bookmark” to assist with transcription or replication of DNA.