During batch growth of Lactococcus lactis subsp. lactis NCDO 2118 on various sugars, the shift from homolactic to mixed-acid metabolism was directly dependent on the sugar consumption rate. This orientation of pyruvate metabolism was related to the flux-controlling activity of glyceraldehyde-3-phosphate dehydrogenase under conditions of high glycolytic flux on glucose due to the NADH/NAD ؉ ratio. The flux limitation at the level of glyceraldehyde-3-phosphate dehydrogenase led to an increase in the pool concentrations of both glyceraldehyde-3-phosphate and dihydroxyacetone-phosphate and inhibition of pyruvate formate lyase activity. Under such conditions, metabolism was homolactic. Lactose and to a lesser extent galactose supported less rapid growth, with a diminished flux through glycolysis, and a lower NADH/NAD ؉ ratio. Under such conditions, the major pathway bottleneck was most probably at the level of sugar transport rather than glyceraldehyde-3-phosphate dehydrogenase. Consequently, the pool concentrations of phosphorylated glycolytic intermediates upstream of glyceraldehyde-3-phosphate dehydrogenase decreased. However, the intracellular concentration of fructose-1,6-bisphosphate remained sufficiently high to ensure full activation of lactate dehydrogenase and had no in vivo role in controlling pyruvate metabolism, contrary to the generally accepted opinion. Regulation of pyruvate formate lyase activity by triose phosphates was relaxed, and mixed-acid fermentation occurred (no significant production of lactate on lactose) due mostly to the strong inhibition of lactate dehydrogenase by the in vivo NADH/NAD ؉ ratio.The industrial importance of lactic acid bacteria is based on their ability to rapidly ferment sugars into lactic acid. For example, metabolism in the homolactic acid bacteria (the model organism is Lactococcus lactis) leads to Ͼ90% conversion of sugars to lactic acid. However, under certain conditions, this homolactic behavior is lost and increased amounts of other metabolites, such as formate or CO 2 , acetate, and ethanol, are produced in what is generally called mixed-acid fermentation. This behavior was first observed in glucose-limited chemostat cultures (22). Homolactic behavior was seen only during rapid growth in which significant amounts of glucose remained in the medium; mixed-acid fermentation was observed at lower rates of growth and true carbon-limited chemostat steady states. Such a mixed metabolism may also occur under carbon-excess conditions with certain sugars. Galactose metabolism of L. lactis results in a fermentation profile in which significant amounts of acetate and ethanol are produced (23), though lactic acid remains the major product (60% of the galactose consumed). A less pronounced shift toward mixed-acid metabolism is also observed during growth on maltose (11, 18). Although details of the biochemical pathways involved remain obscure, the use of pentose sugars involves significant acetate synthesis (9). Under conditions of carbon excess, sugar metabolism in L. lactis o...

Microorganisms extensively reorganize gene expression to adjust growth rate to changes in growth conditions. At the genomic scale, we measured the contribution of both transcription and transcript stability to regulating messenger RNA (mRNA) concentration in Escherichia coli. Transcriptional control was the dominant regulatory process. Between growth rates of 0.10 and 0.63 h−1, there was a generic increase in the bulk mRNA transcription. However, many transcripts became less stable and the median mRNA half-life decreased from 4.2 to 2.8 min. This is the first evidence that mRNA turnover is slower at extremely low-growth rates. The destabilization of many, but not all, transcripts at high-growth rate correlated with transcriptional upregulation of genes encoding the mRNA degradation machinery. We identified five classes of growth-rate regulation ranging from mainly transcriptional to mainly degradational. In general, differential stability within polycistronic messages encoded by operons does not appear to be affected by growth rate. We show here that the substantial reorganization of gene expression involving downregulation of tricarboxylic acid cycle genes and acetyl-CoA synthetase at high-growth rates is controlled mainly by transcript stability. Overall, our results demonstrate that the control of transcript stability has an important role in fine-tuning mRNA concentration during changes in growth rate.