ABSTRACT
Corynebacterium glutamicum is known for its effective excretion of amino acids under particular metabolic conditions. Concomitant activities of uptake and excretion systems would create an energy-wasting futile cycle; amino acid export systems are therefore tightly regulated. We have used a DNA microarray approach to identify genes for membrane proteins which are overexpressed under conditions of elevated cytoplasmic concentrations of methionine. One of these genes was brnF, coding for the larger subunit of BrnFE, a previously identified two-component isoleucine export system. By deletion, complementation, and overexpression of the brnFE genes in a C. glutamicum strain, in which the two uptake systems for methionine were inactivated, we identified BrnFE as being responsible for methionine export. In the presence of both substrates in the cytoplasm, BrnFE was found to transport isoleucine and methionine at similar rates. The expression of the brnFE gene cluster depends on an Lrp-type transcription factor and was shown to be strongly induced by increasing cytoplasmic methionine concentration. Methionine was a better inducer than isoleucine, indicating that methionine rather than isoleucine might be the native substrate of BrnFE. When the synthesis of BrnFE was blocked by chloramphenicol, fast methionine export was still observed, but only at greatly increased cytoplasmic levels of this amino acid. This indicates the presence of at least one other methionine export system, presumably with low affinity but high capacity. Under conditions where cytoplasmic methionine does not exceed a concentration of 50 mM, BrnFE is the dominant export system for this amino acid.
