Identification of a meningococcal L-glutamate ABC transporter operon essential for growth in low-sodium environments

GdhR is a meningococcal transcriptional regulator that was previously shown to positively control the expression of gdhA, encoding the NADP-specific L-glutamate dehydrogenase (NADP-GDH), in response to the growth phase and/or to the carbon source. In this study we used reverse transcriptase-PCR-differential display (to identify additional GdhR-regulated genes. The results indicated that GdhR, in addition to NADP-GDH, controls the expression of a number of genes involved in glucose catabolism by the Entner-Doudoroff pathway and in l-glutamate import by an unknown ABC transport system. The genes encoding the putative periplasmic substrate-binding protein (NMB1963) and the permease (NMB1965) of the ABC transporter were genetically inactivated. Uptake experiments demonstrated an impairment of L-glutamate import in the NMB1965-defective mutant in the absence or in the presence of a low sodium ion concentration. In contrast, at a sodium ion concentration above 60 mM, the uptake defect disappeared, possibly because the activity of a sodium-driven secondary transporter became predominant. Indeed, the NMB1965-defective mutant was unable to grow at a low sodium ion concentration (<20 mM) in a chemically defined medium containing L-glutamate and four other amino acids that supported meningococcal growth, but it grew when the sodium ion concentration was raised to higher values (>60 mM). The same growth phenotype was observed in the NMB1963-defective mutant. Cell invasion and intracellular persistence assays and expression data during cell invasion provided evidence that the l-glutamate ABC transporter, tentatively named GltT, was critical for meningococcal adaptation in the low-sodium intracellular environment.