# VapC10 toxin of the legume symbiont Sinorhizobium meliloti targets tRNASer and controls intracellular lifestyle

## TL;DR

This study shows how a toxin in a soil bacterium helps control its survival inside plant cells during a symbiotic relationship.

## Contribution

The study identifies VapC10 as an RNase toxin targeting tRNA in a symbiotic bacterium, linking it to intracellular lifestyle control.

## Key findings

- VapC10 toxin cleaves the anticodon loop of two tRNASer molecules.
- VapC10 activation reprograms the bacteroid proteome, potentially limiting nitrogen fixation.
- A vapC10 mutant causes delayed nodule senescence and improved bacteroid survival.

## Abstract

The soil bacterium Sinorhizobium meliloti can establish a nitrogen-fixing symbiosis with the model legume Medicago truncatula. The rhizobia induce the formation of a specialized root organ called nodule, where they differentiate into bacteroids and reduce atmospheric nitrogen into ammonia. Little is known on the mechanisms involved in nodule senescence onset and in bacteroid survival inside the infected plant cells. Although toxin-antitoxin (TA) systems have been shown to promote intracellular survival within host cells in human pathogenic bacteria, their role in symbiotic bacteria was rarely investigated. S. meliloti encodes several TA systems, mainly of the VapBC family. Here we present the functional characterization, through a multidisciplinary approach, of the VapBC10 TA system of S. meliloti. Following a mapping by overexpression of an RNase in Escherichia coli (MORE) RNA-seq analysis, we demonstrated that the VapC10 toxin is an RNase that cleaves the anticodon loop of two tRNASer. Thereafter, a bioinformatics approach was used to predict VapC10 targets in bacteroids. This analysis suggests that toxin activation triggers a specific proteome reprogramming that could limit nitrogen fixation capability and viability of bacteroids. Accordingly, a vapC10 mutant induces a delayed senescence in nodules, associated to an enhanced bacteroid survival. VapBC10 TA system could contribute to S. meliloti adaptation to symbiotic lifestyle, in response to plant nitrogen status.