GROWTH-ASSOCIATED SYNTHESIS OF POLY(HYDROXYBUTYRIC ACID) IN METHYLOBACTERIUM-RHODESIANUM AS AN EXPRESSION OF AN INTERNAL BOTTLENECK

Methylobacterium rhodesianum MB126 growing on fructose accumulates pol y(hydroxybutyric acid) (PHB) during exponential growth in contrast to growth on methanol, where the strain accumulates the polyester in resp onse to limitation of several growth factors. A mathematical descripti on of the kinetics, based on the growth-associated term of product for mation, yields a good correlation to the experimental data, as it is v alidated by the course of PHB content after the substrate shift from m ethanol to fructose. From a comparison of carbon fluxes with enzyme ca pacities, we assume a bottleneck in the tricarboxylic acid cycle to be the reason for the growth-associated PHB synthesis. PHB formation fro m fructose, which can be considered as a kind of fermentation, helps o vercome the bottleneck. This hypothesis is supported by the fact that the simultaneous PHB synthesis can be suppressed by an additional supp ly of NADH via the oxidation of formate.