The Cryptosporidium "mouse" genotype is conserved across geographic areas.
A 298-bp region of the Cryptosporidium parvum 18S rRNA gene and a 390-bp region of the acetyl coenzyme A synthetase gene were sequenced for a range of Cryptosporidium isolates from wild house mice (Mus domesticus), a bat (Myotus adversus), and cattle from different geographical areas. Previous research has identified a distinct genotype, referred to as the "mouse"-derived Cryptosporidium genotype, common to isolates from Australian mice. Comparison of a wider range of Australian mouse isolates with United Kingdom and Spanish isolates from mice and cattle and also an Australian bat-derived Cryptosporidium isolate revealed that the "mouse" genotype is conserved across geographic areas. Mice are also susceptible to infection with the "cattle" Cryptosporidium genotype, which has important implications for their role as reservoirs of infection for humans and domestic animals. (+info)
The role of an iron-sulfur cluster in an enzymatic methylation reaction. Methylation of CO dehydrogenase/acetyl-CoA synthase by the methylated corrinoid iron-sulfur protein.
This paper focuses on how a methyl group is transferred from a methyl-cobalt(III) species on one protein (the corrinoid iron-sulfur protein (CFeSP)) to a nickel iron-sulfur cluster on another protein (carbon monoxide dehydrogenase/acetyl-CoA synthase). This is an essential step in the Wood-Ljungdahl pathway of anaerobic CO and CO2 fixation. The results described here strongly indicate that transfer of methyl group to carbon monoxide dehydrogenase/acetyl-CoA synthase occurs by an SN2 pathway. They also provide convincing evidence that oxidative inactivation of Co(I) competes with methylation. Under the conditions of our anaerobic assay, Co(I) escapes from the catalytic cycle one in every 100 turnover cycles. Reductive activation of the CFeSP is required to regenerate Co(I) and recruit the protein back into the catalytic cycle. Our results strongly indicate that the [4Fe-4S] cluster of the CFeSP is required for reductive activation. They support the hypothesis that the [4Fe-4S] cluster of the CFeSP does not participate directly in the methyl transfer step but provides a conduit for electron flow from physiological reductants to the cobalt center. (+info)
Purification and catalytic properties of Ech hydrogenase from Methanosarcina barkeri.
Methanosarcina barkeri has recently been shown to produce a multisubunit membrane-bound [NiFe] hydrogenase designated Ech (Escherichia coli hydrogenase 3) hydrogenase. In the present study Ech hydrogenase was purified to apparent homogeneity in a high yield. The enzyme preparation obtained only contained the six polypeptides which had previously been shown to be encoded by the ech operon. The purified enzyme was found to contain 0.9 mol of Ni, 11.3 mol of nonheme-iron and 10.8 mol of acid-labile sulfur per mol of enzyme. Using the purified enzyme the kinetic parameters were determined. The enzyme catalyzed the H2 dependent reduction of a M. barkeri 2[4Fe-4S] ferredoxin with a specific activity of 50 U x mg protein-1 at pH 7.0 and exhibited an apparent Km for the ferredoxin of 1 microM. The enzyme also catalyzed hydrogen formation with the reduced ferredoxin as electron donor at a rate of 90 U x mg protein-1 at pH 7.0. The apparent Km for the reduced ferredoxin was 7.5 microM. Reduction or oxidation of the ferredoxin proceeded at similar rates as the reduction or oxidation of oxidized or reduced methylviologen, respectively. The apparent Km for H2 was 5 microM. The kinetic data strongly indicate that the ferredoxin is the physiological electron donor or acceptor of Ech hydrogenase. Ech hydrogenase amounts to about 3% of the total cell protein in acetate-grown, methanol-grown or H2/CO2-grown cells of M. barkeri, as calculated from quantitative Western blot experiments. The function of Ech hydrogenase is ascribed to ferredoxin-linked H2 production coupled to the oxidation of the carbonyl-group of acetyl-CoA to CO2 during growth on acetate, and to ferredoxin-linked H2 uptake coupled to the reduction of CO2 to the redox state of CO during growth on H2/CO2 or methanol. (+info)
Molecular cloning and cell-cycle-dependent expression of the acetyl-CoA synthetase gene in Tetrahymena cells.
To identify transcriptionally regulated mediators associated with the cell cycle, we adopted the differential mRNA display technique for cell cultures of Tetrahymena pyriformis synchronized by cyclic heat treatment. One cDNA fragment that was expressed differently during synchronous cell division had a greatly decreased expression at 30 min after the end of heat treatment (EHT). Using this fragment as a probe, we isolated the full-length cDNA for T. pyriformis acetyl-CoA synthetase (TpAcs) which encodes a 651 amino acid polypeptide with a predicted molecular mass of 72.8 kDa. The deduced amino acid sequence of T. pyriformis ACS shows 42% sequence identity compared with that of Lysobacter sp. acetyl-CoA synthetase (ACS), an enzyme which catalyses the formation of acetyl-CoA from acetate via an acetyl-adenylate intermediate. The deduced sequence is also 41% and 40% identical compared with those of Pseudomonas putida and Coprinus cinereus ACS, respectively. The deduced sequence of T. pyriformis ACS also shares similar characteristics of the conserved motifs I and II in the ACS family. To further investigate the actions of the gene encoding this enzyme, mRNA expression was determined during the course of synchronized cell division in T. pyriformis. Northern blot results show that the mRNA level was dramatically decreased at 30 min after EHT prior to entering synchronous cell division (which occurs 75 min after EHT), suggesting that mRNA expression of the TpAcs was associated with the cell cycle and that the down-regulated expression of TpAcs at 30 min after EHT would be required for the initiation of the oncoming synchronous cell division in T. pyriformis. (+info)
Oxalic acid production by Aspergillus niger: an oxalate-non-producing mutant produces citric acid at pH 5 and in the presence of manganese.
The external pH appeared to be the main factor governing oxalic acid production by Aspergillus niger. A glucose-oxidase-negative mutant produced substantial amounts of oxalic acid as long as the pH of the culture was 3 or higher. When pH was decreased below 2, no oxalic acid was formed. The activity of oxaloacetate acetylhydrolase (OAH), the enzyme believed to be responsible for oxalate formation in A. niger, correlated with oxalate production. OAH was purified from A. niger and characterized. OAH cleaves oxaloacetate to oxalate and acetate, but A. niger never accumulated any acetate in the culture broth. Since an A. niger acuA mutant, which lacks acetyl-CoA synthase, did produce some acetate, wild-type A. niger is apparently able to catabolize acetate sufficiently fast to prevent its production. An A. niger mutant, prtF28, previously isolated in a screen for strains deficient in extracellular protease expression, was shown here to be oxalate non-producing. The prtF28 mutant lacked OAH, implying that OAH is the only enzyme involved in oxalate production in A. niger. In a traditional citric acid fermentation low pH and absence of Mn2+ are prerequisites. Remarkably, a strain lacking both glucose oxidase (goxC) and OAH (prtF) produced citric acid from sugar substrates in a regular synthetic medium at pH 5 and under these conditions production was completely insensitive to Mn2+. (+info)
sigma(70) is the principal sigma factor responsible for transcription of acs, which encodes acetyl coenzyme A synthetase in Escherichia coli.
Cells of Escherichia coli undergo a metabolic switch associated with the production and utilization of acetate. During exponential growth on tryptone broth, these cells excrete acetate via the phosphotransacetylase-acetate kinase (Pta-AckA) pathway. As they begin the transition to stationary phase, they instead resorb acetate, activate it to acetyl coenzyme A (acetyl-CoA) by means of the enzyme acetyl-CoA synthetase (Acs) and utilize it to generate energy and biosynthetic components via the tricarboxylic acid cycle and the glyoxylate shunt, respectively. This metabolic switch depends upon the induction of Acs. As part of our effort to dissect the mechanism(s) underlying induction and to identify the signal(s) that triggers that induction, we sought the sigma factor most responsible for acs expression. Using isogenic strains that carry a temperature sensitivity allele of the gene that encodes sigma(70) and either a wild-type or null allele of the gene that encodes sigma(S), we determined by immunoblotting, reverse transcriptase PCR, and acs::lacZ transcriptional fusion analyses that sigma(70) is the sigma factor primarily responsible for the acs transcription that cells induce during mid-exponential phase. In contrast, sigma(S) partially inhibits that transcription as cells enter stationary phase. (+info)
Evidence for intersubunit communication during acetyl-CoA cleavage by the multienzyme CO dehydrogenase/acetyl-CoA synthase complex from Methanosarcina thermophila. Evidence that the beta subunit catalyzes C-C and C-S bond cleavage.
The carbon monoxide dehydrogenase/acetyl-CoA synthase (CODH/ACS) from Methanosarcina thermophila is part of a five-subunit complex consisting of alpha, beta, gamma, delta, and epsilon subunits. The multienzyme complex catalyzes the reversible oxidation of CO to CO(2), transfer of the methyl group of acetyl-CoA to tetrahydromethanopterin (H(4)MPT), and acetyl-CoA synthesis from CO, CoA, and methyl-H(4)MPT. The alpha and epsilon subunits are required for CO oxidation. The gamma and delta subunits constitute a corrinoid iron-sulfur protein that is involved in the transmethylation reaction. This work focuses on the beta subunit. The isolated beta subunit contains significant amounts of nickel. When proteases truncate the beta subunit, causing the CODH/ACS complex to dissociate, the amount of intact beta subunit correlates directly with the EPR signal intensity of Cluster A and the activity of the CO/acetyl-CoA exchange reaction. Our results strongly indicate that the beta subunit harbors Cluster A, a NiFeS cluster, that is the active site of acetyl-CoA cleavage and assembly. Although the beta subunit is necessary, it is not sufficient for acetyl-CoA synthesis; interactions between the CODH and the ACS subunits are required for cleavage or synthesis of the C-C bond of acetyl-CoA. We propose that these interactions include intramolecular electron transfer reactions between the CODH and ACS subunits. (+info)
Acetyl-CoA synthetase from the amitochondriate eukaryote Giardia lamblia belongs to the newly recognized superfamily of acyl-CoA synthetases (Nucleoside diphosphate-forming).
The gene coding for the acetyl-CoA synthetase (ADP-forming) from the amitochondriate eukaryote Giardia lamblia has been expressed in Escherichia coli. The recombinant enzyme exhibited the same substrate specificity as the native enzyme, utilizing acetyl-CoA and adenine nucleotides as preferred substrates and less efficiently, propionyl- and succinyl-CoA. N- and C-terminal parts of the G. lamblia acetyl-CoA synthetase sequence were found to be homologous to the alpha- and beta-subunits, respectively, of succinyl-CoA synthetase. Sequence analysis of homologous enzymes from various bacteria, archaea, and the eukaryote, Plasmodium falciparum, identified conserved features in their organization, which allowed us to delineate a new superfamily of acyl-CoA synthetases (nucleoside diphosphate-forming) and its signature motifs. The representatives of this new superfamily of thiokinases vary in their domain arrangement, some consisting of separate alpha- and beta-subunits and others comprising fusion proteins in alpha-beta or beta-alpha orientation. The presence of homologs of acetyl-CoA synthetase (ADP-forming) in such human pathogens as G. lamblia, Yersinia pestis, Bordetella pertussis, Pseudomonas aeruginosa, Vibrio cholerae, Salmonella typhi, Porphyromonas gingivalis, and the malaria agent P. falciparum suggests that they might be used as potential drug targets. (+info)