Abstract
Wolbachia are common bacteria among terrestrial arthropods. These endosymbionts transmitted through the female germline manipulate their host reproduction through several mechanisms whose most prevalent form called Cytoplasmic Incompatibility -CI- is a conditional sterility syndrome eventually favoring the infected progeny. Upon fertilization, the sperm derived from an infected male is only compatible with an egg harboring a compatible Wolbachia strain, this sperm leading otherwise to embryonic death. The Wolbachia Cif factors CidA and CidB responsible for CI and its neutralization function as a Toxin-Antitoxin system in the mosquito host Culex pipiens. However, the mechanism of CidB toxicity and its neutralization by the CidA antitoxin remain unexplored. Using transfected insect cell lines to perform a structure-function analysis of these effectors, we show that both CidA and CidB are chromatin interactors and CidA anchors CidB to the chromatin in a cell-cycle dependent-manner. In absence of CidA, the CidB toxin localizes to its own chromatin microenvironment and acts by preventing S-phase completion, independently of its deubiquitylase -DUB- domain. Experiments with transgenic Drosophila show that CidB DUB domain is required together with CidA during spermatogenesis to stabilize the CidA-CidB complex. Our study defines CidB functional regions and paves the way to elucidate the mechanism of its toxicity.