Activity of Exporters of <i>Escherichia coli</i> in <i>Corynebacterium glutamicum</i>, and Their Use to Increase <i>L</i>-Threonine Production

L-Threonine is an important biotechnological product and Corynebacterium glutamicum is able to synthesize and accumulate this amino acid to high intracellular levels. We here use four exporters of Escherichia coli and show that three of them operate in C. glutamicum, with RhtA and RhtC being the most effective. Whereas RhtA was unspecific, resulting in L-homoserine together with L-threonine excretion, this was not the case with RhtC. Expression of rhtC reduced the intracellular L-threonine concentration from 140 to 11 mM and resulted in maximal excretion rates of 11.2 nmol min(-1) mg(-1) as compared to 2.3 nmol min(-1) mg(-1) obtained without rhtC expression. In combination with an ilvA mutation generated and introduced into the chromosome, an accumulation of up to 54 mM L-threonine was achieved as compared to 21 mM obtained with the ancestor strain. This shows that expression of rhtC is the pivotal point for industrial relevant L-threonine production with C. glutamicum, and might encourage in general the use of heterologous exporters in the field of white biotechnology to make full use of biosynthesis pathways.