Continuous-Culture Responses of Candida shehatae to Shifts in Temperature and Aeration: Implications for Ethanol Inhibition

Temperature and aeration shifts were used to perturb steady-state continuous cultures to determine the effects of ethanol on xylose metabolism by Candida shehatae. The accumulation of ethanol exerted a delayed inhibitory effect on the specific rate of substrate utilization. A second effect was also observed in which the specific rate of xylitol production increased at the expense of the specific rate of ethanol production. Both effects were enhanced at higher temperature. Inhibitory effects also occurred in glucose metabolism.     

Cleavage-site motifs in mitochondrial targeting peptides

Although mitochondrial targeting peptides lack a common consensus sequence, a certain bias in the positional distribution of amino acids has recently been found. These patterns seem to be associated with cleavage of the precursor proteins by matrix processing proteases. We have extended the previous studies and found new sequence motifs that are conserved within subgroups of mitochondrial targeting peptides. These motifs have certain common themes, indicating that they are associated with cleavage by one single protease. Two of the conserved patterns have a high predictive value, but even for sequences that do not possess these patterns, a fairly accurate prediction of the cleavage site is shown to be possible. We also suggest that a well-conserved RXY decreases (S/A) pattern may be used to engineer efficiently recognized cleavage sites into uncleaved or artificial mitochondrial targeting peptides.     

Aerobic nitrate and nitrite reduction in continuous cultures of Escherichia coli E4

Nitrate and nitrite was reduced by Escherichia coli E4 in a l-lactate (5 mM) limited culture in a chemostat operated at dissolved oxygen concentrations corresponding to 90–100% air saturation. Nitrate reductase and nitrite reductase activity was regulated by the growth rate, and oxygen and nitrate concentrations. At a low growth rate (0.11 h^–1) nitrate and nitrite reductase activities of 200 nmol · mg^–1 protein · min^–1 and 250 nmol · mg^–1 protein · min^–1 were measured, respectively. At a high growth rate (0.55 h^–1) both enzyme activities were considerably lower (25 and 12 nmol mg^–1 · protein · min^–1). The steady state nitrite concentration in the chemostat was controlled by the combined action of the nitrate and nitrite reductase. Both nitrate and nitrite reductase activity were inversely proportional to the growth rate. The nitrite reductase activity decreased faster with growth rate than the nitrate reductase. The chemostat biomass concentration of E. coli E4, with ammonium either solely or combined with nitrate as a source of nitrogen, remained constant throughout all growth rates and was not affected by nitrite concentrations. Contrary to batch, E. coli E4 was able to grow in continuous cultures on nitrate as the sole source of nitrogen. When cultivated with nitrate as the sole source of nitrogen the chemostat biomass concentration is related to the activity of nitrate and nitrite reductase and hence, inversely proportional to growth rate.     

Enzymatic evidence for involvement of the methylmalonyl-CoA pathway in propionate oxidation by Syntrophobacter wolinii

Enzyme measurements were carried out with crude cell-free extracts of the propionate oxidizing coculture of Syntrophobacter wolinii and Desulfovibrio G11. Using cell-free extracts of a pure culture of Desulfovibrio G11 as a blank, most of the enzymes involved in the methylmalonyl-CoA pathway for propionate oxidation, including a propionyl-CoA: oxaloacetate transcarboxylase, were demonstrated in S. wolinii.     

Purification and some properties of the methyl-CoM reductase of Methanothrix soehngenii

Abstract The methyl-CoM reductase from Methanothrix soehngenii was purified 18-fold to apparent homogeneity with 50% recovery in three steps. The native molecular mass of the enzyme estimated by gel-fitration was 280 kDa. SDS-polyacrylamide gel electrophoresis revealed three protein bands corresponding to M _r 63 900, 41 700 and 30 400 Da. The methyl-coenzyme M reductase constitutes up to 10% of the soluble cell protein. The enzyme has K _m apparent values of 23 μM and 2 mM for N -7-mercaptoheptanoylthreonine phosphate (HS- HTP = component B ) and methyl-coenzyme M (CH₃CoM) respectively. At the optimum pH of 7.0 60 nmol of methane were formed per min per mg protein.     

Reductive dechlorination of 1,2-dichloroethane and chloroethane by cell suspensions of methanogenic bacteria

Concentrated cell suspensions of methanogenic bacteria reductively dechlorinated 1,2-dichloroethane via two reaction-mechanisms: a dihalo-elimination yielding ethylene and two hydrogenolysis reactions yielding chloroethane and ethane, consecutively. The transformation of chloroethane to ethane was inhibited by 1,2-dichloroethane. Stimulation of methanogenesis caused an increase in the amount of dechlorination products formed, whereas the opposite was found when methane formation was inhibited. Cells of Methanosarcina barkeri grown on H₂/CO₂ converted 1,2-dichloroethane and chloroethane at higher rates than acetate or methanol grown cells.Abbreviations BrES 2-bromoethanesulfonic acid - CA chloroethane - 1,2-DCA 1,2-dichloroethane - F₄₃₀ Ni(II)tetrahydro-(12, 13)-corphin with an uroporphinoid (III) ligand skeleton     

Vasopressin-enhanced urea transport by rat inner medullary collecting duct cells in culture

Abstract. The distal inner medullary collecting duct (IMCD) is critical in the urinary concentrating process, in part because it is the site of vasopressin (AVP)-regulated permeability to urea. The purpose of these experiments was to develop a cell culture model of the IMCD on permeable structure and to characterize the responsiveness to AVP. Rat IMCD cells were grown to confluence on collagen-coated Millipore filters glued onto plastic rings. To assess the time required to achieve confluence, the transepithelial resistance was measured periodically and was found to be stable after 2 weeks, at a maximal value of 595 ± 22 ω cm². In separate monolayers the effect of AVP on inulin and urea permeability was determined. While inulin permeability was unchanged after AVP, urea permeability increased from 6.0 ± 0–4 to peak values of 16.0 ± 3–8(10nM),23.1 ± 3–9(1 μM)and28 1 ± 4–9(10μM) X 10^-6cms^-1 ( n = 24). In 10 other monolayers, after the addition of 1 mM 8-Br-cAMP, urea permeability increased from 5.1 ±0–3 to 8.1 ± 1–6 times 10^-6 cm s^-1 and, after 8-Br-cAMP +3-isobutyl-l-methylxanthine, to 12.2 ± 0–7 times 10^-6 cms^-1. We conclude that rat IMCD cells grown in culture exhibit the characteristics of a 'tight' epithelium. Inulin and urea permeability are not different in the absence of AVP, consistent with high resistance junctional complexes. Furthermore, IMCD cells retain the capacity for AVP-regulated urea permeability, a characteristic feature of this nephron segment in vivo.