ORCID Profile
0000-0002-2148-3115
Current Organisations
Stellenbosch University
,
Afrigen Biologics & Vaccines
,
Kwela Brews
,
LABiome-Tech
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Publisher: American Society for Microbiology
Date: 08-2003
DOI: 10.1128/AEM.69.8.4534-4541.2003
Abstract: Catabolism of amino acids via the Ehrlich pathway involves transamination to the corresponding α-keto acids, followed by decarboxylation to an aldehyde and then reduction to an alcohol. Alternatively, the aldehyde may be oxidized to an acid. This pathway is functional in Saccharomyces cerevisiae , since during growth in glucose-limited chemostat cultures with phenylalanine as the sole nitrogen source, phenylethanol and phenylacetate were produced in quantities that accounted for all of the phenylalanine consumed. Our objective was to identify the structural gene(s) required for the decarboxylation of phenylpyruvate to phenylacetaldehyde, the first specific step in the Ehrlich pathway. S. cerevisiae possesses five candidate genes with sequence similarity to genes encoding thiamine diphosphate-dependent decarboxylases that could encode this activity: YDR380w/ARO10 , YDL080C/THI3 , PDC1 , PDC5 , and PDC6 . Phenylpyruvate decarboxylase activity was present in cultures grown with phenylalanine as the sole nitrogen source but was absent from ammonia-grown cultures. Furthermore, the transcript level of one candidate gene ( ARO10 ) increased 30-fold when phenylalanine replaced ammonia as the sole nitrogen source. Analyses of phenylalanine catabolite production and phenylpyruvate decarboxylase enzyme assays indicated that ARO10 was sufficient to encode phenylpyruvate decarboxylase activity in the absence of the four other candidate genes. There was also an alternative activity with a higher capacity but lower affinity for phenylpyruvate. The candidate gene THI3 did not itself encode an active phenylpyruvate decarboxylase but was required along with one or more pyruvate decarboxylase genes ( PDC1 , PDC5 , and PDC6 ) for the alternative activity. The K m and V max values of the two activities differed, showing that Aro10p is the physiologically relevant phenylpyruvate decarboxylase in wild-type cells. Modifications to this gene could therefore be important for metabolic engineering of the Ehrlich pathway.
Publisher: Oxford University Press (OUP)
Date: 06-2007
DOI: 10.1111/J.1567-1364.2007.00220.X
Abstract: Aerobic, glucose-limited chemostat cultures of Saccharomyces cerevisiae grown with six different nitrogen sources were subjected to transcriptome analysis. The use of chemostats enabled an analysis of nitrogen-source-dependent transcriptional regulation at a fixed specific growth rate. A selection of preferred (ammonium and asparagine) and nonpreferred (leucine, phenylalanine, methionine and proline) nitrogen sources was investigated. For each nitrogen source, distinct sets of genes were induced or repressed relative to the other five nitrogen sources. In total, 131 such 'signature transcripts' were identified in this study. In addition to signature transcripts, genes were identified that showed a transcriptional coresponse to two or more of the six nitrogen sources. For ex le, 33 genes were transcriptionally upregulated in leucine-grown, phenylalanine-grown and methionine-grown cultures this was partly attributed to the involvement of common enzymes in the dissimilation of these amino acids. In addition to specific transcriptional responses elicited by in idual nitrogen sources, their impact on global regulatory mechanisms such as nitrogen catabolite repression (NCR) were monitored. NCR-sensitive gene expression in the chemostat cultures showed that ammonium and asparagine were 'rich' nitrogen sources. By this criterion, leucine, proline and methionine were 'poor' nitrogen sources, and phenylalanine showed an 'intermediate' NCR response.
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Siew Leng Tai.