Differing Roles for Clostridium acetobutylicum’s Galactose Utilization Pathways

There has been a surge of interest in acetone-butanol-ethanol fermentations of Clostridium acetobutylicum due to its capacity to ferment many carbohydrates found in biomass. This metabolic diversity makes it a promising candidate for conversion of inexpensive, heterogeneous carbohydrate feedstocks to biofuels. Galactose is present in many such feedstocks due to its incorporation in plant cell walls. C. acetobutylicum encodes two galactose utilization pathways, the Leloir (LP) and the tagatose-6-P (T6P), and a previous study indicated genes for these pathways was differentially regulated during growth on galactose and lactose. In the current study we utilized quantitative PCR to further investigate gene expression levels and to show both pathways which were subject to carbon catabolite repression. During growth on galactose, mRNA for galactose-6-P isomerase from the T6P was induced to a greater extent than mRNA for glactokinase, the first enzyme in the LP. The galactose-6-P isomerase mRNAs were also more abundant than galactokinase mRNAs during growth on galactose. Analysis of theoretical ATP requirements to generate essential precursor metabolites indicated: 1) the LP is more efficient at generating upper glycolytic intermediates, 2) the T6P is more efficient at forming ATP, lower glycolytic intermediates and TCA cycle intermediates, 3) a combination of the two pathways is most efficient for forming precursor metabolites found in the pentose phosphate pathway. From this it can be suggested that the two pathways have different roles in the organism with the T6P generating most ATP and precursor metabolites and the LP providing upper glycolytic metabolites.