Search

Acetic acid-mediated inhibition of the fermentation of lignocellulose-derived sugars impedes development of plant biomass as a source of renewable ethanol. In order to overcome this inhibition, the capacity of Saccharomyces cerevisiae to synthesize acetyl-CoA from acetic acid was increased by overexpressing ACS2 encoding acetyl-coenzyme A synthetase. Overexpression of ACS2 resulted in higher resistance to acetic acid as measured by an increased growth rate and shorter lag phase relative to a wild-type control strain, suggesting that Acs2-mediated consumption of acetic acid during fermentation contributes to acetic acid detoxification., (© FEMS 2014. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

The genome sequence of Roseovarius sp. strain 217 indicated that many pathway enzymes found in other organisms for the degradation of taurine are represented, but that a novel, apparently energy-dependent pathway is involved in the conversion of acetyl phosphate to acetyl CoA. Thus, an ABC transporter for taurine could be postulated, while inducible taurine: pyruvate aminotransferase, alanine dehydrogenase, sulfoacetaldehyde acetyltransferase and sulfite dehydrogenase could be assayed. Whereas phosphate acetyltransferase has been found in other organisms, none was indicated in the genome sequence and no activity was found in cell-free extracts. Instead, acetate kinase was active as was acetate-CoA ligase.

The gene (acs) encoding the acetyl-CoA synthetase (Acs) in Pseudomonas putida U has been cloned, sequenced and expressed in different microbes. The protein has been purified and characterized from a biochemical, structural and evolutionary point of view. Disruption or deletion of acs handicapped the bacterium for growth in a chemically defined medium containing acetate; this ability was regained when P. putida U was transformed with a plasmid carrying this gene. By contrast, all the acs knock-out mutants could assimilate n-alkanoic acids having a carbon length greater than C2, suggesting that other acyl-CoA activating enzymes (different from Acs) are involved in the catabolism of these compounds. However, these enzymes that can replace the function played by Acs in vivo are not induced by acetate.

Mutants in Aspergillus niger unable to grow on acetate as a sole carbon source were previously isolated by resistance to 1.2% propionate medium containing 0.1% glucose. AcuA mutants lacked acetyl-CoA synthetase (ACS) activity and acuB mutants lacked both ACS and isocitrate lyase activity. An acuA mutant was transformed to the acu+ phenotype with a clone of ACS (facA) from Aspergillus nidulans. The acuB mutant was transformed with the A. niger facB clone which has been identified by cross-hybridisation of an A. nidulans facB clone. These results confirm that acuA in A. niger is the gene for ACS and acuB is analogous to the A. nidulans facB regulatory gene.

In Saccharomyces cerevisiae, the structural genes ACS1 and ACS2 each encode an isoenzyme of acetyl-CoA synthetase (ACS; EC 6.2.1.1). Involvement of glucose catabolite repression in regulation of the two isoenzymes was investigated by following ACS activity after glucose pulses (100 mM) to ethanol-limited chemostat cultures. In wild-type S. cerevisiae and in an isogenic strain in which ACS2 had been disrupted, ACS activity decreased after a glucose pulse. No such inactivation was observed in a strain in which ACS1 was disrupted. Western blots demonstrated that the ACS1 product, but not the ACS2 product, was degraded after a glucose pulse. Inactivation kinetics of the ACS1 product resembled those of isocitrate lyase.

Two independent pathways in Escherichia coli convert acetate to acetyl CoA: reversal of acetate production by phosphotransacetylase and acetate kinase, and the acetyl-CoA synthetase (Acs) pathway that scavenges acetate. We investigated acs gene expression by using a cat transcriptional fusion. It was observed that acs expression varies depending on the carbon sources used and occurs in the stationary phase of growth even in the absence of acetate. Mutations in iclR for the repressor of the glyoxylate shunt and in rpoS for the stationary phase sigma factor reduced the consumption of acetate mediated by Acs, indicating that both are involved in acs regulation.