Mycobacterium tuberculosis Rv2700 Contributes to Cell Envelope Integrity and Virulence

The cell envelope of Mycobacterium tuberculosis is a key target for antibiotics, yet its assembly and maintenance remain incompletely understood. Here we report that Rv2700, a previously uncharacterized M. tuberculosis gene, contributes to envelope integrity. Specifically, an Rv2700 mutant strain had a decreased growth rate, increased sensitivity to antibiotics that target peptidoglycan crosslinking, and increased cell envelope permeability. We propose that Rv2700 be named a "cell envelope integrity" gene (cei). Importantly, a cei mutant had attenuated virulence in mice. Cei shares predicted structural homology with another M. tuberculosis protein, VirR (Rv0431), and we found that a virR mutant had growth rate, antibiotic sensitivity, and envelope permeability phenotypes similar to those of the cei mutant. Both Cei and VirR are predicted to consist of a transmembrane helix and an extracellular LytR_C domain. LytR_C domains have no known function, but they are also found in a family of proteins, the LytR-Cps2A-Psr (LCP) enzymes, that perform important cell envelope functions in a range of bacteria. In mycobacteria, LCP enzymes attach arabinogalactan to peptidoglycan, and mycobacterial LCP enzyme mutants have phenotypes similar to those of virR- and cei-deficient strains. Collectively, our results suggest that LytR_C domain proteins may contribute to the cell envelope functions performed by LCP proteins. This study provides a framework for further mechanistic investigations of LytR_C proteins and, more broadly, for advancing our understanding of the cell envelopes of mycobacteria and other medically and economically important genera.IMPORTANCEMycobacterium tuberculosis causes about 1.5 million deaths per year. The unique composition of the Mycobacterium tuberculosis cell envelope is required for this bacterium to cause disease and is the target for several critical antibiotics. By better understanding the mechanisms by which mycobacteria assemble and maintain their cell envelope, we might uncover new therapeutic targets. In this work, we show that a previously uncharacterized protein, Rv2700, is important for cell envelope integrity in Mycobacterium tuberculosis and that loss of Rv2700 attenuates virulence in mice. This family of proteins is found in a broad group of bacterial species, so our work provides a first insight into their potential functions in many species important to the environment, industry, and human health.