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Oxygen-Mediated Regulation of Porphobilinogen Formation in Rhodobacter capsulatus. Alan J. Biel, 2002.A Rhodobacter capsulatus hemC mutant has been isolated and used to show that oxygen regulates the intracellular levels of porphobilinogen . Experiments using a hemB-cat gene fusion demonstrated that oxygen does not transcriptionally regulate hemB transcription . Porphobilinogen synthase activity is not regulated by oxygen nor is the enzyme feedback inhibited by hemin or protoporphyrin IX . It was demonstrated that less than 20% of [14C]aminolevulinate was incorporated into bacteriochlorophyll, suggesting that the majority of the aminolevulinate is diverted from the common tetrapyrrole pathway . Porphobilinogen oxygenase activity was not observed in this organism; however, an NADPH-linked aminolevulinate dehydrogenase activity was demonstrated . The specific activity of this enzyme increased with increasing oxygen tension . The results presented here suggest that carbon flow over the common tetrapyrrole pathway is regulated by a combination of feedback inhibition of aminolevulinate synthase and diversion of aminolevulinate from the pathway by aminolevulinate dehydrogenase . Ethanol Tolerance in the Yeast Saccharomyces cerevisiae Is Dependent on Cellular Oleic Acid Content. Kyung Man You, 2003.In this investigation, we examined the effects of different unsaturated fatty acid compositions of Saccharomyces cerevisiae on the growth-inhibiting effects of ethanol . The unsaturated fatty acid (UFA) composition of S . cerevisiae is relatively simple, consisting almost exclusively of the mono-UFAs palmitoleic acid (  9Z-C16:1) and oleic acid (9Z-C18:1), with the former predominating . Both UFAs are formed in S . cerevisiae by the oxygen- and NADH-dependent desaturation of palmitic acid (C16:0) and stearic acid (C18:0), respectively, catalyzed by a single integral membrane desaturase encoded by the OLE1 gene . We systematically altered the UFA composition of yeast cells in a uniform genetic background (i) by genetic complementation of a desaturase-deficient ole1 knockout strain with cDNA expression constructs encoding insect desaturases with distinct regioselectivities (i.e.,
9 and
11) and substrate chain-length preferences (i.e., C16:0 and C18:0); and, (ii) by supplementation of the same strain with synthetic mono-UFAs . Both experimental approaches demonstrated that oleic acid is the most efficacious UFA in overcoming the toxic effects of ethanol in growing yeast cells . Furthermore, the only other UFA tested that conferred a nominal degree of ethanol tolerance is cis-vaccenic acid (11Z-C18:1), whereas neither
11Z-C16:1 nor palmitoleic acid (9Z-C16:1) conferred any ethanol tolerance . We also showed that the most ethanol-tolerant transformant, which expresses the insect desaturase TniNPVE, produces twice as much oleic acid as palmitoleic acid in the absence of ethanol and undergoes a fourfold increase in the ratio of oleic acid to palmitoleic acid in response to exposure to 5% ethanol . These findings are consistent with the hypothesis that ethanol tolerance in yeast results from incorporation of oleic acid into lipid membranes, effecting a compensatory decrease in membrane fluidity that counteracts the fluidizing effects of ethanol .
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