Regulatory Phenomena in the Metabolism of Knallgasbacteria

Advanced PHB fermentation strategies with CO2-derived organic acids

Over the past decade, formic acid and acetic acid have gained increasing attention as alternative feedstocks for poly-3-hydroxybutyrate (PHB) production as these potentially CO₂-derived molecules are naturally assimilated by Cupriavidus necator. Both organic acids were individually evaluated in fed-batch fermentations at bioreactor scale. Acetic acid was revealed as the most promising carbon source yielding 42.3 g L^-1 PHB, whereas no significant amount of PHB was produced from formic acid. Hence, acetic acid was further used as the substrate during process intensification. Key performance characteristics, including process stability, PHB titer, and productivity were optimized by introducing NH₄-acetate as the nitrogen source, extending the growth phase, and implementing a repeated fed-batch procedure, respectively. These advanced fermentation strategies resulted in the establishment of a stable fermentation process reaching 58.5 g L^-1 PHB, while doubling the productivity to 0.93 g L^-1 h^-1 PHB.

Regulation of the histidine utilization (hut) system in bacteria

The ability to degrade the amino acid histidine to ammonia, glutamate, and a one-carbon compound (formate or formamide) is a property that is widely distributed among bacteria. The four or five enzymatic steps of the pathway are highly conserved, and the chemistry of the reactions displays several unusual features, including the rearrangement of a portion of the histidase polypeptide chain to yield an unusual imidazole structure at the active site and the use of a tightly bound NAD molecule as an electrophile rather than a redox-active element in urocanase. Given the importance of this amino acid, it is not surprising that the degradation of histidine is tightly regulated. The study of that regulation led to three central paradigms in bacterial regulation: catabolite repression by glucose and other carbon sources, nitrogen regulation and two-component regulators in general, and autoregulation of bacterial regulators. This review focuses on three groups of organisms for which studies are most complete: the enteric bacteria, for which the regulation is best understood; the pseudomonads, for which the chemistry is best characterized; and Bacillus subtilis, for which the regulatory mechanisms are very different from those of the Gram-negative bacteria. The Hut pathway is fundamentally a catabolic pathway that allows cells to use histidine as a source of carbon, energy, and nitrogen, but other roles for the pathway are also considered briefly here.

Nitrogen fixation and hydrogen metabolism in cyanobacteria

This review summarizes recent aspects of (di)nitrogen fixation and (di)hydrogen metabolism, with emphasis on cyanobacteria. These organisms possess several types of the enzyme complexes catalyzing N(2) fixation and/or H(2) formation or oxidation, namely, two Mo nitrogenases, a V nitrogenase, and two hydrogenases. The two cyanobacterial Ni hydrogenases are differentiated as either uptake or bidirectional hydrogenases. The different forms of both the nitrogenases and hydrogenases are encoded by different sets of genes, and their organization on the chromosome can vary from one cyanobacterium to another. Factors regulating the expression of these genes are emerging from recent studies. New ideas on the potential physiological and ecological roles of nitrogenases and hydrogenases are presented. There is a renewed interest in exploiting cyanobacteria in solar energy conversion programs to generate H(2) as a source of combustible energy. To enhance the rates of H(2) production, the emphasis perhaps needs not to be on more efficient hydrogenases and nitrogenases or on the transfer of foreign enzymes into cyanobacteria. A likely better strategy is to exploit the use of radiant solar energy by the photosynthetic electron transport system to enhance the rates of H(2) formation and so improve the chances of utilizing cyanobacteria as a source for the generation of clean energy.

Kinetics as a tool for polyhydroxyalkanoate production optimization

The increasing commercial importance of polyhydroxyalkanoates calls for the development of new, more efficient production processes. This can only be achieved by considering the kinetics of polyhydroxyalkanoate accumulation in fermentors, but efforts in this area have been few. In a 10-L fed-batch fermentor, Alcaligenes eutrophus G+3was used to produce poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P(3HB-co-4HB)) from glucose and γ-butyrolactone, and a strain of Alcaligenes latus was used to produce poly(3-hydroxybutyrate-co-3-hydroxyvalerate) from glucose and propionate. After 83 h of fermentation, 9.26 g∙L−1of A. eutrophus contained 77.8% in mass of a copolymer with 7.9 mol% 4HB. The observed maximum specific growth rate (μmax) was 0.19 h−1for the residual biomass. Alcaligenes latus grew at an observed residual-biomass μmaxof 0.41 h−1and after 33.75 h had produced 6.6 g∙L−1of dry biomass with 72% of a copolymer with 28 mol% 3-hydroxyvalerate. Yields and specific substrate consumption and product formation rates were calculated. Examination of these results and of data found in the literature led to the proposition that for certain polyhydroxyalkanoate production processes, a multi-stage system consisting of a continuous stirred-tank fermentor in series with a plug-flow tubular reactor would be the most productive solution.Key words: polyhydroxyalkanoate, Alcaligenes latus, Alcaligenes eutrophus, kinetics, optimization.

Regulation of H2 oxidation activity and hydrogenase protein levels by H2, O2, and carbon substrates in Alcaligenes latus

Regulation of H2 oxidation activity and hydrogenase protein levels in the free-living hydrogen bacterium Alcaligenes latus was investigated. Hydrogenase activity was induced when heterotrophically grown cells were transferred to chemolithoautotrophic conditions, i.e., in the presence of H2 and absence of carbon sources, with NH4Cl as the N source. Under these conditions, H2 oxidation activity was detectable after 30 min of incubation and reached near-maximal levels by 12 h. The levels of hydrogenase protein, as measured by a Western blot (immunoblot) assay of the hydrogenase large subunit, increased in parallel with activity. This increase suggested that the increased H2 oxidation activity was due to de novo synthesis of hydrogenase protein. H2 oxidation activity was controlled over a surprisingly wide range of H2 concentrations, between 0.001 and 30% in the gas phase. H2 oxidation activity was induced to high levels between 2 and 12.5% O2, and above 12.5% O2, H2 oxidation activity was inhibited. Almost all organic carbon sources studied inhibited the expression of hydrogenase, although none repressed hydrogenase synthesis completely. In all cases examined, hydrogenase protein, as detected by Western blot, paralleled the level of H2 oxidation activity, suggesting that control of hydrogenase activity was mediated through changes in hydrogenase protein levels.

Regulation of two nickel-requiring (inducible and constitutive) hydrogenases and their coupling to nitrogenase in Methylosinus trichosporium OB3b

Two uptake hydrogenases were found in the obligate methanotroph Methylosinus trichosporium OB3b; one was constitutive, and a second was induced by H2. Both hydrogenases could be assayed by measuring methylene blue reduction anaerobically or by coupling their activity to nitrogenase acetylene reduction activity in vivo in an O2-dependent reaction. The H2 concentration for half-maximal activity of the inducible and constitutive hydrogenases in both assays was 0.01 and 0.5 bar (1 and 50 kPa), respectively, making it easy to distinguish these enzymes from one another both in vivo and in vitro. Hydrogen uptake was shown to be coupled to ATP synthesis in methane-starved cells. Methane, methanol, formate, succinate, and glucose all repressed the H2-mediated synthesis of the inducible hydrogenase. Furthermore, this enzyme was only expressed in N-starved cultures and was repressed by NH4+ and NO3-; synthesis of the constitutive hydrogenase was not affected by excess N in the growth medium. In nickel-free, EDTA-containing medium, the activities of these two enzymes were negligible; however, both enzyme activities appeared rapidly following the addition of nickel to the culture. Chloramphenicol, when added along with nickel, had no effect on the rapid appearance of either the constitutive or inducible activity, indicating that nickel is not required for synthesis of the hydrogenase apoproteins. These observations all suggest that these hydrogenases are nickel-containing enzymes. Finally, both hydrogenases were soluble and could be fractionated by 20% ammonium sulfate; the constitutive enzyme remained in the supernatant solution, while the inducible enzyme was precipitated under these conditions.

Coordinate expression of hydrogenase and ribulose bisphosphate carboxylase in Rhizobium japonicum Hupc mutants

In contrast to the wild type, H2 uptake-constitutive mutants of Rhizobium japonicum expressed both hydrogenase and ribulose bisphosphate carboxylase activities when grown heterotrophically. However, as bacteroids from soybean root nodules, the H2 uptake-constitutive mutants, like the wild type, did not express ribulose bisphosphate carboxylase activity.

Mutagenesis of Alcaligenes eutrophus by insertion of the drug-resistance transposon Tn5

Drug-resistance element Tn5 coding for kanamycin resistance was used for mutagenesis of Alcaligenes eutrophus strain H16. The vehicle for introducing Tn5 into A. eutrophus was plasmid pJB4JI harboured by Escherichia coli. Kanamycin-resistant transconjugants occurred at a frequency of approximately 5 x 10(-8). One third of the transconjugants exhibited other plasmid-coded resistances such as gentamycin and spectinomycin. However, the latter markers were not stably maintained in the new host. Among the kanamycin-resistant transconjugants three classes of mutants were found: (i) Auxotrophic mutants occurred at a frequency of 0.8% and showed requirements for histidine, methionine, aspartate or isoleucine. Out of eleven auxotrophic mutants examined eight reverted to prototrophy. However, none of the revertants was kanamycin-sensitive. (ii) Mutants unable to grow with fructose as the carbon source occurred at a frequency of almost 10%. (iii) Mutants which had lost the ability to grow autotrophically with hydrogen and carbon dioxide were found at a frequency of 1%. Further analyses revealed that this class of mutants was either defective in carbon dioxide fixation or impaired in hydrogen metabolism.

Nickel requirement for active hydrogenase formation in Alcaligenes eutrophus

The nickel-dependent chemolithoautotrophic growth of Alcaligenes eutrophus is apparently due to a requirement of nickel for active hydrogenase formation. Cells grown heterotrophically with fructose and glycerol revealed a specific activity of soluble and membrane-bound hydrogenase which was severalfold higher than the normal autotrophic level. The omission of nickel from the medium did not affect heterotrophic growth, but the soluble hydrogenase activity was reduced significantly. In the presence of ethylenediaminetetraacetic acid (EDTA), almost no hydrogenase activity was detected. The addition of nickel allowed active hydrogenase formation even when EDTA was present. When chloramphenicol was added simultaneously with nickel to an EDTA-containing medium, almost no hydrogenase activity was found. This indicates that nickel ions are involved in a process which requires protein synthesis and not the direct reactivation of a preformed inactive protein. The formation of the membrane-bound hydrogenase also appeared to be nickel dependent. Autotrophic CO2 assimilation did not specifically require nickel ions, since formate was utilized in the presence of EDTA and the activity of ribulosebisphosphate carboxylase was not affected under these conditions.

Hydrogenase genes

For a variety of reasons, including the potential industrial applications of hydrogenase, we are interested in the isolation and analysis of hydrogenase genes. In a program focusing on the hydrogen bacterium A. eutrophus H1 and E. coli, we have developed a preliminary concept of the interaction of hydrogenase in cellular metabolism, constructed mutants deficient in hydrogenase activity, and begun the isolation of hydrogenase genes utilizing the technology allowing the in vitro manipulation of DNA. We hope to pursue this project to its ultimate goal: the analysis of the molecular mechanisms involved in the control of expression of these genes and the development of the ability to manipulate the production of hydrogenase.

Aerobic hydrogenase activity in Anacystis nidulans. The oxyhydrogen reaction

1. The oxyhydrogen reaction of Anacystis nidulans was studied manometrically and polarographically in whole cells and in cell-free preparations; the activity was found to be associated with the particulate fraction. 2. Besides O2, the isolated membranes reduced artificial electron acceptors of positive redox potential; the reactions were unaffected by O2 levels less than 10--15%; aerobically the artificial acceptors were reduced simultaneously with O2. 3. H2-supported O2 uptake was inhibited by CO, KCN and 2-n-heptyl-8-hydroxyquinoline-N-oxide. Inhibition by CO was partly reversed by strong light. Uncouplers stimulated the oxyhydrogen reaction. 4. The kinetic properties of O2 uptake by isolated membranes were the same in presence of H2 and of other respiratory substrates. 5. Low rates of H2 evolution by the membrane preparations were found in presence of dithionite; methyl viologen stimulated the reaction. 6. The results indicate that under certain growth conditions Anacystis synthesizes a membrane-bound hydrogenase which appears to be involved in phosphorylative electron flow from H2 to O2 through the respiratory chain.

Purification and properties of the particulate hydrogenase from the bacteroids of soybean root nodules

The uptake hydrogenase (hydrogen:ferricytochrome c3 oxidoreductase, EC 1.12.2.1) from the bacteroids of soybean root nodules infected with Rhizobium japonicum 110 has been purified and characterized. Bacteroids were prepared, then broken by sonication. The particulate enzyme was solubilized by treatment with Triton X-100 and further purified by polyethylene glycol fractionation, DEAE-cellulose and Sephadex G-100 chromatography. The specific activity has been increased 196-fold to 19.6 units/mg protein. The molecular weight is 63 300 as determined by gel filtration and 65 300 as determined by SDS-polyacrylamide gel electrophoresis, indicating that the enzyme is a monomer. The enzyme is O2 sensitive, with a half-life of 70 min when exposed to air. The pH optimum of the solubilized enzyme is near 5.5; the Km for H2 is 1.4 microM. Suitable electron acceptors are methylene blue, ferricyanide, 2,6-dichlorophenolindophenol, and cytochrome c. Benzyl viologen is reduced slowly; methyl viologen, NAD(P)+, FAD, FMN, and O2 are not reduced. The optimum temperature for activity is 65-70 degrees C with an activation energy of 9.2 kcal. H2 evolution by the enzyme has been demonstrated. The hydrogenase is well-suited to function in an environment where all the available H2 is generated in situ.

Regulation of hydrogenase in Rhizobium japonicum

Factors that regulate the expression of an H2 uptake system in free-living cultures of Rhizobium japonicum have been investigated. Rapid rates of H2 uptake by R. japonicum were obtained by incubation of cell suspensions in a Mg-phosphate buffer under a gas phase of 86.7% N2, 8.3% H2, 4.2% CO2, and 0.8% O2. Cultures incubated under conditions comparable with those above, with the exception that Ar replaced H2, showed no hydrogenase activity. When H2 was removed after initiation of hydrogenase derepression, further increase in hydrogenase activity ceased. Nitrogenase activity was not essential for expression of hydrogenase activity. All usable carbon substrates tested repressed hydrogenase formation, but none of them inhibited hydrogenase activity. No effect on hydrogenase formation was observed from the addition of KNO3 or NH4Cl at 10 mM. Oxygen repressed hydrogenase formation, but did not inhibit activity of the enzyme in whole cells. The addition of rifampin or chloramphenicol to derepressed cultures resulted in inhibition of enzyme formation similar to that observed by O2 repression. The removal of CO2 during derepression caused a decrease in the rate of hydrogenase formation. No direct effect of CO2 on hydrogenase activity was observed.

Hydrogen metabolism in aerobic hydrogen-oxidizing bacteria

A survey on organisms able to use molecular hydrogen as electron donor in the energy-yielding process is presented. In the group of the aerobic hydrogen-oxidizing bacteria so far two types of hydrogenases have been encountered, a NAD-reducing, soluble enzyme (H2 : NAD oxidoreductase) and a membrane-bound enzyme unable to reduce pyridine nucleotides. With respect to the distribution of both types of hydrogenases three groups of hydrogen-oxidizing bacteria can be diffentiated containing (i) both types (Alcaligenes eutrophus), (ii) a soluble enzyme only (Nocardia opaca lb), and (iii) a membrane-bound hydrogenase only (majority of genera and species). The results of studies on the NAD-specific hydrogenase of A. eutrophus are summarized. Results on the solubilization and purification of the membrane-bound hydrogenase of A. eutrophus are presented in detail. The enzyme was solubilized from purified membranes by Triton X-100 and sodium desoxycholate or phospholipase D. The crude membrane extract was fractionated by ammonium sulfate precipitation and chromatography on carboxymethylcellulose at pH 5.5. The enzyme was stable in potassium phosphate buffer; it resembles the soluble enzyme with respect to stability under oxidizing conditions. Further biochemical and immunological data indicate, however, that both enzymes are different with respect to their native structure.

Mutants of Alcaligenes eutrophus defective in autotrophic metabolism

Forty-four mutants of Alcaligenes eutrophus H 16 were isolated which grew poorly or not at all under autotrophic conditions. Four types were characterized with respect to their defects and their physiological properties. One mutant lacked both enzymes specific for autotrophic CO2 fixation, another one lacked both hydrogenases, and two mutants lacked either the membrane-bound or the soluble hydrogenase. Comparing the results of studies on these mutant types, the following conclusions were drawn: the lack of each hydrogenase enzyme could be partially compensated by the other one; the lack of membrane-bound hydrogenase did not affect autotrophic growth, whereas the lack of the soluble hydrogenase resulted in a decreased autotrophic growth rate. When pyruvate as well as hydrogen were supplied to the wild-type, the cell yield was higher than in the presence of pyruvate alone. Mutant experiments under these conditions indicated that either of both hydrogenases was able to add to the energy supply of the cell. Only the soluble hydrogenase was involved in the control of the rate of hydrogen oxidation by carbon dioxide; the mutant lacking this enzyme did not respond to the presence or absence of CO2. The suppression of growth on fructose by hydrogen could be mediated by either of both hydrogenases alone.

The properties of hydrogenase from Thiocapsa roseopersicina

The soluble and chromatophore-bound hydrogenases from the purple sulphur bacterium Thiocapsa roseopersicina strain BBS were purified up to homogeneity and the properties studied. The amino acid composition of hydrogenase preparations from different fractions of T. roseopersicina is identical, glycine and alanine as N-terminal amino acid residues. In comparison with other hydrogenases, especially in the immobilized state, the preparations obtained are shown to be more stable to O2 during storage and they are characterized by high thermal stability. Inactivation is observed above 78--80 degrees C and the optimal temperature for enzyme action is 70 degrees C. The homogeneous enzyme preparations catalyse the exchange reaction between 2H2 and H2O and reversible redox reactions of methyl viologen and benzyl viologen as well as H2 formation from reduced ferredoxin. According to our data, the hydrogenase of T. roseopersicina bound with chromatophores is identical to the soluble one.

Hydrogenase and ribulose diphosphate carboxylase during autotrophic, heterotrophic, and mixotrophic growth of scotochromogenic mycobacteria

Two key autotrophic enzyme systems, hydrogenase and ribulose diphosphate carboxylase, were examined in Mycobacterium gordonae and two other chemolithotrophic, scotochromogenic mycobacteria under different cultural conditions. In all three organisms both enzymes were inducible and were produced in significant levels only in the presence of the specific substrate, hydrogen or carbon dioxide. M. gordonae exhibited increased growth rates and yields, indicating mixotrophic growth, in the presence of a number of single organic substrates, including acetate, pyruvate, glucose, fructose, and glycerol. In contrast to other aerobic hydrogen autotrophs, the presence of either acetate or pyruvate did not repress ribulose diphosphate carboxylase, and mixotrophic growth was rapid with these substrates. In the absence of carbon dioxide, growth in glycerol medium under an atmosphere of hydrogen and oxygen was severely inhibited, even with cells preadapted to heterotrophic growth on glycerol. Cyclic adenosine monophosphate was not effective in inducing hydrogenase or carboxylase in heterotrophic, mixotrophic, or hydrogen-inhibited cultures.

Catabolism of D-fructose and D-ribose by Pseudomonas doudoroffii. I. Physiological studies and mutant analysis

Pseudomonas doudoroffii, a strict aerobe of marine origin, was able to utilize fructose and ribose but not glucose, gluconate, or other hexoses, pentoses, or sugar alcohols as sole sources of carbon and energy. Evidence was presented indicating that in this organism fructose was utilized via an inducible P-enolpyruvate: fructose phosphotransferase system (FPTS) which catalyzed the phosphorylation of fructose in the 1 position. The resulting fructose-1-P (F-1-P) was converted to fructose-1,6-P2 (FDP) by means of an inducible 1-P-fructokinase (1-PFK). The subsequent conversion of FDP to pyruvate involved enzymes of the Embden-Meyerhof pathway (EMP) which, with the exception of glyceraldehyde-3-P dehydrogenase (G3PDH), were constitutive. Two G3PDH activities were detected, one of which was inducible and NAD-dependent while the other was constitutive and NADP-dependent. Cell-free extracts of P. doudoroffii also contained enzymes of the methylglyoxal pathway (MGP) which converted dihydroxyacetone-P to pyruvate. The low specific activities of enzymes of this pathway as compared to the EMP suggested that the major route of FDP catabolism was via the latter pathway. 2. Ribose catabolism appeared to involve an inducible uptake system and an inducible ribokinase, the resulting ribose-5-P being converted to glyceraldehyde-3-P and fructose-6-P (F-6-P) by means of constitutive activities of the pentose-P pathway. The F-6-P formed as a result of these reactions was converted to FDP by means of a constitutive 6-P-fructokinase (6-PFK). Since no activity converting fructose or F-1-P to F-6-P could be detected in cell-free extracts of P. doudoroffii, the results suggested that fructose and ribose were catabolized via 1-PFK and 6-PFK, respectively, the two pathways converging at the level of FDP. Further evidence for this suggestion was obtained from a mutant which lacked an NAD-dependent G3PDH, accumulated FDP from both fructose and ribose, and was not able to grow on either of these compounds. 3. Ribose grown cells had increased amounts of the fructose uptake system and 1-PFK suggesting that a compound (or compounds) common to the catabolism of both fructose and ribose acted as the inducer(s) of these activities. Evidence was presented suggesting that the probable inducer(s) of 1-PFK and FPTS could be FDP, glyceraldehyde-3-P, or dihydroxyacetone-P. 4. A mutant unable to grow on fructose was characterized and found to lack FPTS while retaining 1-PFK and other enzyme activities of the EMP and MGP, indicating that a functional FPTS was essential for growth on fructose and suggesting that all or most of this sugar was catabolized via F-1-P.

Leucine biosynthesis in the blue-green bacterium Anacystis nidulans

Leucine-requiring auxotrophs of the unicellular blue-green bacterium Anacystis nidulans have been isolated. Extracts of these mutants were deficient in alpha-isopropylmalate synthetase (EC 4.1.3.12). In wild-type cells, this enzyme was subject to feedback inhibition by leucine. However, formation of the enzymes of leucine biosynthesis was little affected by exogenous leucine in either wild-type or mutant strains. Cultures of the latter subjected to extreme leucine deprivation showed no change in specific activity of beta-isopropylmalate isomerase (EC 4.2.1.33) and at most a 50% increase in the specific activity of beta-isopropylmalate dehydrogenase (EC 1.1.1.85). These results are compared with others bearing on the evolution of the control of amino acid biosynthesis in blue-green bacteria.

Regulation of the Thiobacillus intermedius glucose uptake system by thiosulfate

Cells of the mixotrophic chemolithotroph (facultative autotroph) Thiobacillus intermedius which have been grown on a glucose-yeast extract medium, a condition in which glucose is used as a source of energy, accumulate the non-metabolizable analogue 2-deoxy-d-glucose against a concentration gradient in a predominantly unchanged state. On the other hand, cells grown mixotrophically on a thiosulfate-glucose medium, a condition in which glucose provides cell carbon but is not used extensively for energy, and in which enzymes of the Entner-Doudoroff pathway are repressed, do not accumulate 2-deoxy-d-glucose significantly. Similarly, cells grown chemolithotrophically on thiosulfate-carbonate do not take up this sugar. Transfer of thiosulfate-yeast extract-grown cells, which lack the capacity to accumulate 2-deoxy-d-glucose, to a glucose-yeast extract medium results in the induction of the concentrative sugar uptake system. The capacity of induced cells to take up 2-deoxy-d-glucose is inhibited by thiosulfate. Thus, the transport system for glucose appears to be regulated in this organism so that the sugar is accumulated only under conditions where it is utilized as a source of energy, and the presence of the preferred energy source leads to both repression and inhibition of the uptake system.

Transport and catabolism of D-fructose by Spirillum itersomii

Spirillum itersonii ATCC 12639 utilized d-fructose but neither d-glucose nor d-gluconate as a sole source of carbon and energy. The substrate saturation kinetics for d-fructose and d-glucose uptake by whole cells indicated the presence of a carrier-mediated transport system for d-fructose but not for d-glucose. The d-fructose uptake activity was induced (10- to 12-fold increase) during growth on d-fructose-Casamino Acids (CA) or d-glucose-CA medium, but not CA alone. d-Fructose uptake activity was stimulated by Na(+) or Li(+), but was inhibited by KCN, NaN(3), 2,4-dinitrophenol, and p-chloromercuribenzoate. High specific activities of glucokinase, glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydratase, and 2-keto-3-deoxy-6-phosphogluconate aldolase were detected in extracts of cells cultured on d-fructose-CA medium. These enzymatic activities were undetectable in extracts of cells grown in CA or succinate-CA medium. No decrease in the maximally induced specific activities of these enzymes occurred after the addition of succinate to cells during exponential growth on d-fructose-CA. Fructose 1,6-diphosphate aldolase and glucose-6-phosphate isomerase specific activities were approximately the same irrespective of cultural conditions. These results indicated that d-glucose was not utilized by cells of S. itersonii because this bacterium was impermeable to this hexose.