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

<i>L</i>-Threonine is an important biotechnological product and <i>Corynebacterium glutamicum</i> is able to synthesize and accumulate this amino acid to high intracellular levels. We here use four exporters of <i>Escherichia coli</i> and show that three of them operate in <i>C. glutamicum</i>, with RhtA and RhtC being the most effective. Whereas RhtA was unspecific, resulting in <i>L</i>-homoserine together with <i>L</i>-threonine excretion, this was not the case with RhtC. Expression of <i>rhtC</i> reduced the intracellular <i>L</i>-threonine concentration from 140 to 11 m<i>M</i> 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 <i>rhtC</i> expression. In combination with an <i>ilvA</i> mutation generated and introduced into the chromosome, an accumulation of up to 54 m<i>M</i><i>L</i>-threonine was achieved as compared to 21 m<i>M</i> obtained with the ancestor strain. This shows that expression of <i>rhtC</i> is the pivotal point for industrial relevant <i>L</i>-threonine production with <i>C. glutamicum</i>, and might encourage in general the use of heterologous exporters in the field of white biotechnology to make full use of biosynthesis pathways.