The cell wall barrier of Corynebacterium glutamicum and amino acid efflux

Corynebacterium glutamicum is extremely well suited for the production of a mino acids, and the way in which the biosynthesis pathways have to be engin eered for this purpose is very well understood. However, the special signif icance of the cell envelope as a barrier for the production process is only just being recognized. In addition to the pathways it determines the cellu lar synthesis capacity. The cell wall of the Corynebacterianeae, which also include Mycobacterium tuberculosis, has a complex structure and first deta iled findings on the structure and synthesis of their cell wall are availab le. In addition to the ubiquitous inner lipid bilayer, the cell envelope ha s an outer lipid layer which contains mycolic acids and is probably also or ganized as a bilayer. During export, the amino acid has to pass these diffe rent layers of the cell wall. Molecular investigations have now identified the L-lysine exporter LysE and the L-threonine exporter ThrE which are loca lized in the inner cytoplasmic bilayer. It was revealed that both carriers represent the prototype of previously unknown translocator families. This i nvolves extended families whose members are present in bacteria and archeae . The L-lysine exporter also exports L-arginine. Its expression is regulate d by an elevated concentration of the cell-internal amino acid, which may, for example, be the case in the presence of peptides. Export thus represent s 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 sp ecial and surprisingly different treatments, such as the addition of deterg ents, 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 o f these different additions is primarily the outer mycolic acid layer. The individual twenty amino acids might pass the various layers of the cell env elope in quite different ways. A major challenge for future work is to disc over how this takes place in detail and to then apply these findings for a further strain improvement.