Saccharomyces cerevisiae Gene Expression in Response to Saxitoxin

 

Microarray expression profiling is becoming a common tool in the field of ecotoxicogenomics to determine the mechanism of action of specific toxicants and to establish “genetic signatures” for identification of the toxicant. Aquatic systems throughout the world have been increasingly plagued in recent years by harmful algal blooms (HABs), the species of which produce toxins that pose threats to multiple trophic levels within the ecosystem, and also to human health. The genetic fingerprint obtained with recent microarray analyses with the yeast Saccharomyces cerevisiae identified a set of copper and iron homeostasis genes as being significantly differentially expressed upon exposure to saxitoxin, a potent neurotoxin produced by several species of dinoflagellates and cyanobacteria found in both marine and freshwater systems. As copper and iron homeostasis are tightly linked in S. cerevisiae, the aim of this recent work was to compare the transcriptional regulation in a subset of genes upon exposure to saxitoxin, excess copper, and excess iron. Gene expression was examined with quantitative reverse-transcriptase PCR using gene-specific primers and TaqmanTM probes. Expression levels were measured using absolute quantification following normalization to the housekeeping gene ACT1. Four of the eight functional genes examined were differentially expressed (relative to controls) in all treatments: the metallothioneins CUP1 and CRS5 were significantly upregulated, with greatest induction occurring upon exposure to copper, while the copper transporter CTR1 and cupric/ferric reductase FRE1 were significantly repressed. Expression of the multicopper oxidase FET3, which remained unchanged upon exposure to copper, was significantly down-regulated in both saxitoxin and iron treatments. Comparing the transcriptional response of metal-associated genes in S. cerevisiae upon exposure to saxitoxin resulted in a similar yet distinct profile to that of copper and an identical profile to that of iron, with differences among treatments occurring at the level (i.e. fold-change) of regulation. Using expression profiling to measure environmental contaminants such as toxins provides an understanding of how toxins influence biological systems. This information is essential to developing better predictive capabilities of water quality, allowing for the intelligent development of water quality indicators such as biomarkers or reporter gene assays. In addition, it provides insights into the ecological functions of the organisms that produce these toxins.
 

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