Metabolic and regulatory properties of sulfide oxidation in Chlorobaculum tepidum
Date
2016
Authors
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Publisher
University of Delaware
Abstract
Hydrogen sulfide has a multifaceted way of interacting with life. To humans and some aerobic organisms, sulfide is toxic at low concentrations, but it also serves as the primary energy source for chemolithoautotrophic based ecosystems like those found at hydrothermal vents, hot springs, and sulfidic caves. Sulfide is produced from the anaerobic degradation of organic matter and is a byproduct of, or waste material from, various industrial operations. Therefore, the development of methods for removing hydrogen sulfide from waste streams is important. Microorganisms that oxidize sulfide, such as the Chlorobi, provide insights into understanding how sulfide can fuel microbial growth in various environments, which could help develop efficient methods for treating sulfide containing waste streams. Chlorobaculum (Cba.) tepidum, an anaerobic photoautotroph preferentially uses sulfide as an electron donor in anoxygenic photosynthesis to fix carbon dioxide into biomass. Three homologs of sulfide:quinone oxidoreductase (SQR), the key sulfide oxidizing enzyme, are encoded by the genome of Cba. tepidum: CT0117, CT0876, and CT1087. SQR can be divided into six types. Each SQR homolog from Cba. tepidum is affiliated with a phylogenetically distinct type of SQR: type IV, type V, and type VI, respectively. Of the three homologs, CT1087 appears to be adapted to high sulfide concentrations since it is required for growth at high sulfide concentrations and is expressed in a sulfide-dependent manner. This dissertation investigates what the physiological roles of each SQR homolog are, and how Cba. tepidum may regulate the sulfide oxidation system. Here I will describe the purification and characterization of a high sulfide adapted SQR homolog encoded by CT1087. Unlike other characterized SQRs, CT1087 has a low affinity for sulfide and high turnover rate. I will also describe a phenotype of a Cba. tepidum strain in which the non-functioning homolog of SQR, encoded by CT0876, was mutagenized. Without CT0876, the sulfide uptake rate of Cba. tepidum is significantly reduced at micromolar sulfide concentrations. The growth of the CT0876 mutant lagged behind the wild type, suggesting it plays a role in increasing the efficiency of sulfide oxidation. Finally, I will describe the role of the putative transcription regulator, encoded by CT1277, in regulating the expression of CT1087. A CT1277 mutant strain of Cba. tepidum expressed CT1087 at significantly higher levels than observed in the wild type indicating that CT1277 functioned as a repressor of CT1087. However, CT1087 expression was still increased in the CT1277 mutant strain after sulfide exposure. These results expand on our understanding of SQR functional diversity by characterizing the first type VI SQR, CT1087, as well as describe a phenotype that suggests a potential functional for a non-sulfide-oxidizing type V SQR, CT0876. These results also identify one of the first transcription regulators involved in sulfide-dependent gene regulation.