The production of acetate by aerobically growing E. coli is examined. The problem is formulated in terms of a flow network that has as its objective maximal ATP synthesis. It is found that when loads are imposed and flux constraints exist either at the level of NADH turnover rate or the activity of a key Krebs cycle enzyme, switching to acetate overflow is predicted. Moreover, the result found for the latter constraint can be shown to be formally equivalent to a correlation experimentally determined for the specific rate of acetate production by E. coli K-12.

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A computer model is described which is capable of predicting changes in cell composition, cell size, cell shape, and the timing of chromosome synthesis in response to changes in external glucose limitation. The model is constructed primarily from information on unrestricted growth in glucose minimal medium. The ability of the model to make reasonable quantitative predictions under glucose-limitation is a test of the plausibility of the basic biochemical mechanisms included in the model. Such a model should be of use in differentiating among competing hypotheses for biological mechanisms and in suggesting as yet unobserved phenomena. The last two points are illustrated with the testing of a mechanism for the control of the initiation of DNA synthesis and predictions on cellwidth variations during the division cycle.

We examine the effect of poly(ethylene glycol) (PEG) on pyrene solubilization behaviors in aqueous sodium dodecyl sulfate (SDS) solutions. These solutions display strong polymer-surfactant complexation. Following the definitions of Ikeda and Maruyama (J. Colloid Interface Sci. 1994, 166, 1) we distinguish between the macroscopic solubilizing power and the microscopic solubilization capacity. With pyrene as a model solubilizate, we use ultraviolet absorbance spectrophotometry to measure solubilizing powers. We use excimer fluorescence spectroscopy to identify polymer-surfactant binding transitions and to measure the aggregation numbers of free SDS micelles and of PEG-bound SDS aggregates that contain solubilized pyrene in order to calculate solubilization capacities. The solubilization capacity and solubilizing power of free SDS micelles both increase with increasing aggregation number, when the aggregation number is increased by increasing ionic strength. The solubilization capacity is approximately 3 times more sensitive than the solubilizing power to a change in aggregation number. For a given value of the ionic strength, the aggregation number of a PEG-bound SDS aggregate is approximately 50-60% smaller than that of a free micelle, while its solubilization capacity is within approximately 20% of that of a free micelle. As a result, PEG increases the macroscopic solubilizing power at all SDS concentrations above the critical aggregation concentration by virtue of the greater number of distinct surfactant aggregates formed for a given SDS concentration in the presence of PEG. Compared to free micelles, the solubilizing power of PEG-bound SDS aggregates is significantly more sensitive to ionic strength.

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We investigated whether the yield of the B vitamin folic acid could be elevated in Bacillus subtilis. Strategies for increasing the folic acid yield were investigated by employing computer-aided flux analysis and mutation. Controlling the activity of the enzyme pyruvate kinase by placing it under inducible control was one strategy devised to elevate yield while insuring that a rapid growth rate results. Other single mutation strategies included amplifying the expression of the genes in the folate operon and overexpressing the Escherichia coli aroH gene, which encodes 2-dehydro-3-deoxyphosphoheptonate aldolase. The latter could conceivably elevate the abundance of the folic acid precursor, para-aminobenzoic acid. Strains that combined two or more mutations were also constructed. Overall, a strain possessing inducible pyruvate kinase, overexpressed aroH, and increased transcription and translation of genes from the folic operon exhibited the best yield. The yield was eightfold higher than that displayed by the parent B. subtilis 168 strain.

Based on measurements and theoretical analyses, we identified deletion of pyruvate kinase (PYK) activity as a possible route for elimination of acid formation in Bacillus subtilis cultures grown on glucose minimal media. Evidence consistent with the attenuation of PYK flux has come from metabolic flux calculations, metabolic pool and enzymatic activity measurements, and a series of nuclear magnetic resonance experiments, all suggesting a nearly complete inhibition of PYK activity for glucose-citrate fed cultures in which the amount of acid formation was nearly zero. In this paper, we report the construction and characterization of a pyk mutant of B. subtilis. Our results demonstrate an almost complete elimination of acid production in cultures of the pyk mutant in glucose minimal medium. The substantial reduction in acid production is accompanied by increased CO 2 production and a reduced rate of growth. Metabolic analysis indicated a dramatic increase in intracellular pools of phosphoenolpyruvate (PEP) and glucose-6-P in the pyk mutant. The high concentrations of PEP and glucose-6-P could explain the decreased growth rate of the mutant. The substantial accumulation of PEP does not occur in Escherichia coli pyk mutants. The very high concentration of PEP which accumulates in the B. subtilis pyk mutant could be exploited for production of various aromatics.Acid production is among the important factors that limit process stability and cell concentration and thus cell-based biotechnological processes (e.g., see references 14, 23, and 25). Numerous approaches have been used in an attempt to reduce acid formation. One mechanism involves maintaining low levels of glucose in fed batch reactors (24,27). While this can lead to increased cell mass and product formation, it is a capitalintensive method. Manipulation of the growth media might also be used to reduce acid formation relative to the amount of glucose consumed (13,24).Majewski and Domach (17) used a constrained network analysis of the main metabolic pathways in conjunction with reported measurements of enzymatic activity levels to explain acid production by bacterial cells. This analysis suggested that Escherichia coli and Bacillus subtilis have excess glycolytic capacity relative to the Krebs cycle. This idea is consistent with the notion that given all the anabolic and catabolic tasks that metabolic networks must perform, stoichiometric conflicts and other conflicts arise, leading to imperfect coordination of all tasks. It is thus an overflow or "spilling" of excess carbon that leads to acid production.In experiments using B. subtilis to test the overflow hypothesis, it was found that a small amount of citrate added to glucose minimal medium (0.1 mol of citrate/1 mol of glucose) caused the rate of glucose (or total carbon) use per cell to decline several-fold, while the maximal growth rate was not diminished (12). Further, acid production was undetectable in the glucose-citrate cultures. Subsequent work showed that productivity of recombinant protein (units o...