Corynebacterium glutamicum is extremely well suited for the production of amino acids, and the way in which the biosynthesis pathways have to be engineered for this purpose is very well understood. However, the special significance of the cell envelope as a barrier for the production process is only just being recognized. In addition to the pathways it determines the cellular synthesis capacity. The cell wall of the Corynebacterianeae, which also include Mycobacterium tuberculosis, has a complex structure and first detailed findings on the structure and synthesis of their cell wall are available. In addition to the ubiquitous inner lipid bilayer, the cell envelope has an outer lipid layer which contains mycolic acids and is probably also organized as a bilayer. During export, the amino acid has to pass these different layers of the cell wall. Molecular investigations have now identified the L-lysine exporter LysE and the L-threonine exporter ThrE which are localized in the inner cytoplasmic bilayer. It was revealed that both carriers represent the prototype of previously unknown translocator families. This involves extended families whose members are present in bacteria and archeae. The L-lysine exporter also exports L-arginine. Its expression is regulated by an elevated concentration of the cell-internal amino acid, which may, for example, be the case in the presence of peptides. Export thus represents a new bacterial mechanism for regulating the cellular amino acid balance. The export of L-glutamic acid is still enigmatic, although the outer lipid layer seems to play a major role in the efflux of this amino acid. Very special and surprisingly different treatments, such as the addition of detergents, but also the addition of penicillin, are always required in order to obtain high efflux of L-glutamate. It is assumed that the ultimate target of these different additions is primarily the outer mycolic acid layer. The individual twenty amino acids might pass the various layers of the cell envelope in quite different ways. A major challenge for future work is to discover how this takes place in detail and to then apply these findings for a further strain improvement.
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