ABSTRACT
After the discovery that Corynebacterium glutamicum can easily be influenced to excrete L-glutamate, powerful producers were rapidly developed for this amino acid and for L-lysine, too. Mutants of C. glutamicum producing these biotechnologically important compounds are also available for several other amino acids [2,47,66,126]. Whereas the key features for the regulation and individual steps of amino acid biosynthesis have been intensively studied, the analysis and understanding of export
has not initially kept pace. One reason is undoubtedly that it was simply not conceivable that specific export carriers exist for this purpose. It is therefore not surprising that as the first idea for L-glutamate excretion it was assumed that some leakage (diffusion) across the cell membrane might occur [76,108]. Later, the idea of the functional inversion of a glutamate-uptake system mediating L-glutamate transport in the opposite direction was put forward [17]. In the case of L-lysine excretion, another idea was also discussed. Here it was suggested that efflux might be mediated by some osmotically controlled pores [69]. The functional inversion of uptake systems is still under discussion, at least for the ABC transporter HisF of Salmonella typhimurium mediating the ATP-coupled histidine uptake, which is thought to additionally enable excretion of its substrate [45]. This speculative model, however, does not hold for L-glutamate export in C. glutamicum, since a mutant deleted of the ATP-dependent GluABCD uptake system is unaltered in its L-glutamate export activity [62].
