Expression of genes of lipid synthesis and altered lipid composition modulates L-glutamate efflux of Corynebacterium glutamicum

Abstract.

L-Glutamate is made with Corynebacterium glutamicum on a scale of more than 10⁶ tons/year. Nevertheless, formation of this amino acid is enigmatic and there is very limited molecular information available to unravel the apparently complex conditions leading to L-glutamate efflux. Here, we report the isolation and overexpression of the genes involved in lipid synthesis: acp, fadD15, cma, cls, pgsA2, cdsA, gpsA, and plsC, and the inactivation of cma and cls. In addition, the consequences for phospholipid content, temperature sensitivity, as well as detergent-independent and detergent-dependent L-glutamate efflux were quantified. An in part strong alteration of the phospholipid composition was achieved; for instance, overexpression of fadD15 encoding an acyl-CoA ligase resulted in an increase of phosphatidyl inositol from 12.6 to 30.2%. All strains, except that overexpressing acp (acyl carrier protein), exhibited increased temperature sensitivity, with the strongest sensitivity present upon cls (cardiolipin synthetase) inactivation. As a consequence of the genetically modified lipid synthesis, L-glutamate efflux changed quite dramatically; for instance, overexpression of plsC (acylglycerolacyl transferase) resulted in a detergent-triggered increase of L-glutamate accumulation from 92 mM to 108 mM, whereas acp overexpression reduced the accumulation to 24 mM. With some of the overexpressed genes, substantial L-glutamate excretion even without detergent addition was obtained when the fermentation temperature was elevated. These data show that the chemical and physical properties of the cytoplasmic membrane are altered and suggest that this is a necessary precondition to achieve L-glutamate efflux.