A <i>Wolbachia</i> nuclease and its binding partner provide a distinct mechanism for cytoplasmic incompatibility.

Abstract

<i>Wolbachia</i> are endosymbiotic bacteria that infect nearly half of all arthropod species. This pandemic is due in part to their ability to increase their transmission through the female germline, most commonly by a mechanism called cytoplasmic incompatibility (CI). The <i>Wolbachia cid</i> operon, encoding 2 proteins, CidA and CidB, the latter a deubiquitylating enzyme (DUB), recapitulates CI in transgenic <i>Drosophila melanogaster</i> However, some CI-inducing <i>Wolbachia</i> strains lack a DUB-encoding <i>cid</i> operon; it was therefore proposed that the related <i>cin</i> operon codes for an alternative CI system. Here we show that the <i>Wolbachia cin</i> operon encodes a nuclease, CinB, and a second protein, CinA, that tightly binds CinB. Recombinant CinB has nuclease activity against both single-stranded and double-stranded DNA but not RNA under the conditions tested. Expression of the <i>cin</i> operon in transgenic male flies induces male sterility and embryonic defects typical of CI. Importantly, transgenic CinA can rescue defects in egg-hatch rates when expressed in females. Expression of CinA also rescues CinB-induced growth defects in yeast. CinB has 2 PD-(D/E)xK nuclease domains, and both are required for nuclease activity and for toxicity in yeast and flies. Our data suggest a distinct mechanism for CI involving a nuclease toxin and highlight the central role of toxin-antidote operons in <i>Wolbachia</i>-induced cytoplasmic incompatibility.