The bacterial antiviral system DRT10 uses an RNA template to make DNA repeats, much as telomerase extends chromosome ends, and its enzyme shares a branch of the evolutionary tree with telomerase

The bacterial antiviral system DRT10 uses an RNA template to make DNA repeats, much as telomerase extends chromosome ends, and its enzyme shares a branch of the evolutionary tree with telomerase

On October 12, Samuel Sternberg’s laboratory at Columbia University published a paper in Nature on the bacterial antiviral system DRT10. Its enzyme, a reverse transcriptase, uses an RNA template to build a DNA strand from identical repeating units.

The laboratory had already studied two related defense enzymes, DRT2 and DRT9: both repeatedly copy a segment of their RNA into DNA. The next was DRT10, which is often found near defense genes in the genome. Graduate student Stephen Tang found DNA strands consisting of a nine-base unit repeated in tandem in Escherichia coli carrying DRT10. The template consists of nine RNA bases, immediately followed by a copy of its first four bases.

The enzyme copies the nine bases into DNA; the DNA strand detaches from the RNA and pairs with the adjacent copy, positioning the enzyme to copy the template again. In the preprint of this study, five of twenty RNA base substitutions around the template disabled defense, and all five disrupted the match between the template and the adjacent copy. Changing either the start of the template or the adjacent copy alone eliminates repeat production; making matching changes to both restores it, with a new repeat sequence.

Telomerase works in a similar way: cell division shortens telomeres, the protective ends of chromosomes, and telomerase extends them with repeats by repeatedly copying a short segment of its own RNA. In both enzymes, a folded RNA segment marks the template boundary (a hairpin in DRT10), and the growing DNA strand detaches from the template and pairs with the RNA again to begin another cycle. When the template from human telomerase RNA was inserted into DRT10 RNA, the enzyme produced TTAGGG telomeric repeats in both E. coli and human cells.

The three-dimensional structure of the complex shows two enzyme molecules on opposite sides of a single RNA molecule, each with its own template. Only the enzyme whose template is followed by a copy of its beginning builds a long strand; the other produces short fragments. When the RNA segments containing the templates were swapped, the other enzyme began building the long strand. The distance between two RNA hairpins sets the template boundaries: when two bases were removed between one hairpin and the template, the repeat unit shortened from nine bases to three. The authors of a preprint from Tianjin Medical University found the same two-enzyme complex in another DRT10 variant.

As Sternberg recalls, the idea that DRT10 and telomerase might share an ancestor came during a talk by Mario Rodríguez Mestre of the University of Copenhagen, who presented a preliminary evolutionary tree of all reverse transcriptases. “I interrupted Mario: I was really intrigued by a turquoise band of eukaryotic reverse transcriptases that clustered very closely with the bacterial ones.” That band turned out to be telomerase. Telomerase had previously been thought to descend from enzymes of mobile elements, stretches of DNA that can move within the genome, but in a tree of 6734 reverse transcriptases, it fell on the same branch as bacterial defense enzymes. The authors propose that the ability to make DNA repeats from an RNA template arose in a common ancestor of bacteria and eukaryotes, before eukaryotes acquired linear chromosomes.

The laboratory is testing whether extending telomeres can protect cells from aging and hopes to determine whether DRT10 could eventually be used to treat diseases involving impaired telomere maintenance. “This is still early work, but we are excited by the possibility of turning a bacterial defense enzyme into a tool that restores the protective DNA at chromosome ends,” Sternberg says.