Amino Acid Transport and Metabolism in Mycobacteria: Cloning, Interruption, and Characterization of an l -Arginine/γ-Aminobutyric Acid Permease in Mycobacterium bovis BCG

Abstract

Genes encoding l-arginine biosynthetic and transport proteins have been shown in a number of pathogenic organisms to be important for metabolism within the host. In this study we describe the cloning of a gene (Rv0522) encoding an amino acid transporter fromMycobacterium bovis BCG and the effects of its deletion onl-arginine transport and metabolism. The Rv0522 gene of BCG was cloned from a cosmid library by using primers homologous to therocE gene of Bacillus subtilis, a putative arginine transporter. A deletion mutant strain was constructed by homologous recombination with the Rv0522 gene interrupted by a selectable marker. The mutant strain was complemented with the wild-type gene in single copy. Transport analysis of these strains was conducted using ¹⁴C-labeled substrates. Greatly reduced uptake of l-arginine and γ-aminobutyric acid (GABA) but not of lysine, ornithine, proline, or alanine was observed in the mutant strain compared to the wild type, grown in Middlebrook 7H9 medium. However, when the strains were starved for 24 h or incubated in a minimal salts medium containing 20 mM arginine (in which even the parent strain does not grow),l-[¹⁴C]arginine uptake by the mutant but not the wild-type strain increased strongly. Exogenousl-arginine but not GABA, lysine, ornithine, or alanine was shown to be toxic at concentrations of 20 mM and above to wild-type cells growing in optimal carbon and nitrogen sources such as glycerol and ammonium. l-Arginine supplied in the form of dipeptides showed no toxicity at concentrations as high as 30 mM. Finally, the permease mutant strain showed no defect in survival in unactivated cultured murine macrophages compared with wild-type BCG.