Characterization of relA and codY mutants of Listeria monocytogenes:: identification of the CodY regulon and its role in virulence

Listeria monocytogenes is a Gram-positive intracellular parasite and the causative organism of human listeriosis. In this article we demonstrate that L. monocytogenes encodes a functional member of the CodY family of global regulatory proteins that is responsive to both GTP and branched chain amino acids. By transcript analyses we identified the CodY regulon in L. monocytogenes and demonstrated that it comprises genes involved in amino acid metabolism, nitrogen assimilation as well as genes involved in sugar uptake and incorporation, indicating a role for CodY in L. monocytogenes in both carbon and nitrogen assimilation. A Delta relA mutation reduced expression of the CodY regulon in early stationary phase and introduction of a Delta codY mutation into a Delta relA strain restored virulence. These data indicate that the avirulence of the Delta relA mutant can in part be explained by the continued repression of the CodY regulon. The phenotypes of Delta relA and Delta codY mutants were studied in J774.A1 and Caco-2 cells and the Delta relA mutation shown to effect intracellular growth. These results provide the first direct evidence that the activity of a CodY-type protein influences pathogenesis and provides new information on the physiological adaptation of L. monocytogenes to post-exponential phase growth and virulence.