The branched chain aminotransferase IlvE promotes growth, stress resistance, and pathogenesis of Listeria monocytogenes
The bacterial plasma membrane is a key interface during pathogen-host interactions, and membrane composition enhances resistance against host antimicrobial defenses. Branched chain fatty acids (BCFAs) are the major plasma membrane component in the intracellular Gram-positive pathogen Listeria monocytogenes (Lm) and BCFA metabolism is essential for Lm growth and virulence. BCFA synthesis requires branched chain amino acids (BCAAs), and the BCAA Isoleucine (Ile) is a necessary substrate for the predominant membrane anteiso-BCFAs (ai-BCFAs) as well as an environmental signal for virulence regulation in Lm. In this study, we explored how two proteins that metabolize or sense Ile contribute to Lm growth, BCFA metabolism, and virulence. The IlvE aminotransferase incorporates Ile into ai-BCFAs, while CodY is an Ile-sensing regulator that coordinates BCAA synthesis and virulence gene expression. Analysis of deletion mutants lacking IlvE ({Delta}ilvE) or CodY ({Delta}codY) revealed a major role for IlvE under nutrient restriction and stress conditions. Cultures of the {Delta}ilvE mutant contained proportionally less ai-BCFAs relative to wild type, while of the {Delta}codY mutant had a lower proportion of ai-BCFAs in stationary phase, despite containing more cell-associated Ile. Both {Delta}ilvE and {Delta}codY mutants required exogenous Ile for optimal growth, but the {Delta}ilvE mutant had an absolute requirement for Valine and Leucine when Ile was absent. IlvE was also necessary for resistance to membrane stress, cell-to-cell spread, infection of primary macrophages, and virulence in mice. Our findings implicate IlvE as an integral aspect of Lm stress resistance and emphasize the central importance of Ile in Lm growth and virulence.