Core Concepts: How diversity-generating retroelements promote mutation and adaptation in myriad microbes.

Adaptation, a cornerstone of evolutionary change, is rarely straightforward. Acquiring a random mutation that promotes survival can take generations. Prokaryotes such as bacteria and Archaea, along with the viruses they harbor, have compact genomes, leaving them with a limited repertoire of DNA to respond to environmental change. Fifteen years ago, microbiologist Jeffrey Miller at the University of California, Los Angeles and colleagues identified a type of jumping gene known as a retrotransposon in a handful of bacteria and viruses that allowed them to mutate certain surface proteins by using an enzyme called a reverse transcriptase (1). This enzyme can make DNA using a strand of RNA as a template; but unlike the reverse transcriptase found in retroviruses such as HIV, the enzyme, known as a diversity-generating retroelement (DGR), was error prone. At certain locations in the RNA sequence, the reverse transcriptase inserted random DNA nucleotides. The result was a protein that differed ever so slightly from its progenitor. The process could be repeated over and over, even within the same cell, giving microbes an almost unlimited array of variations. “It’s a system that’s beautifully constructed to be constantly evolving in a random but targeted fashion,” Miller says. “The cell can constantly be optimizing its surface proteins.” For well over a decade, scientists thought that DGRs only cropped up occasionally on the prokaryotic tree of life. A recent study in Nature Microbiology reveals that, far from being a rarity, DGRs are commonplace, affecting both bacteria and their Archaeal cousins (2). This Swiss Army Knife of proteins first elucidated by Miller now appears to provide an important andwidely cooptedmeans of survival in an ever-changing world. Investigating DGRs