The amino acid producing bacterium Corynebucferirun glufumicum accumulated lactate, succinate and acetate under oxygen-limited growth conditions. Significant restructuring of carbon flux through the central metabolic pathways occurred with a notable decrease in pentose pathway flux and the operation of the TCA cycle in a reductive mode. Simultaneous consumption of residual sugar and organic acids took place when oxygen sufficient conditions were restored though amino acids yields were signiticantly perturbed. lntrodllction For several decades, Corynebacterium glutamicum or related species, have been used industrially for the production of various amino acids. Under certain conditions in which cell membrane permeability is modified via biotine limitation or surfactant addition these bacteria accumulate glutamic acid to high concentrations in the medium broth (Kikuchi and Nakao, 1986). Empirical optimisation of the fermentation strategy has enabled the final concentrations achieved to exceed 100 g/l while traditional genetic techniques have enabled the range of amino acids produced under appropriate fermentation conditions to be extended. Further progress to improve either product yields or specific production rates would now seem to depend on the capacity to modify carbon flux distribution within the central metabolic network to overcome specific limitations. A major limitation to the pragmatic application of this type of metabolic engineering approach is the relative absence of any real data concerning the regulatory mechanisms controlling carbon distribution through the diverse pathways of intermediary metabolism. The stoechiometric modelling techniques used by Vallino & Stephanopoulos (1991) postulated a high carbon flux through the pen&se pathway (5560% of carbon flux from glucose-6-P) during glucose catabolism: figure recently confirmed by NTvIR studies (A Guyonvarch, pers comm). The central metabolism of C.gluramicum is therefore somewhat different to the 'normal metabolism' model, based mostly on E. co/i drtti.in which carbon flux through the pentose pathway is generally believed to be only 1540% of the total G6P available. Such a difference has been attributed to the apparent absence of the NADmADP transhydrogenase enzyme in C. glutamicum and hence the necessity for a direct production of NADPH to satisfy the requirement for anabolic reactions. While such a metabolism is well adapted to the biotechnologists' requirements it must also be born in mind that such modified carbon flux implies a greatly modified control structure and hence the need to establish a number of regulatory details before the rational improvement of the micro-organism can proceed along defined directives. At the moment, much of 449

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