LYSINE UPTAKE AND EXCHANGE IN CORYNEBACTERIUM-GLUTAMICUM

Resting cells of Corynebacterium glutamicum (ATCC 13032) accumulate [14C]lysine by a transport system with a relatively high affinity (10 μM) and a low maximum velocity (0.15 nmol/min per mg [dry weight]). Uptake of lysine was not inhibited by uncouplers or by ionophores affecting the ion gradients and the energetic state of the cell. Analysis of intracellular amino acid concentrations during the transport reaction as well as kinetic studies revealed that the observed uptake of lysine in fact represents a homologous antiport between extracellular [14C]lysine and intracellular unlabeled lysine. Intracellular [14C]lysine could only be released by the addition of unlabeled lysine to the bacterial suspension. In contrast to this homologous antiport reaction, we observed net uptake of lysine in lysine-depleted cells of a lysine auxotrophic strain. This net uptake was found to be electrogenic and could also be observed as a heterologous antiport reaction in wild-type cells under particular conditions. In this case exchange was mediated between internal lysine and external alanine, isoleucine, or valine. This antiport was electrogenic, since the substrates differ in charge. The cells can switch between electroneutral homologous exchange and electrogenic heterologous antiport mode during fermentation because of changing metabolic conditions.