Metabolism of Inorganic Sulfur Compounds in Purple Bacteria

Abstract

This chapter focuses on dissimilatory and assimilatory metabolism of inorganic sulfur compounds by the bacteriochlorophyll-containing purple bacteria. Many anoxygenic phototrophic purple bacteria use inorganic sulfur compounds (e.g., sulfide, elemental sulfur, polysulfides, thiosulfate, or sulfide) as electron donors for reductive carbon dioxide fixation during photolithoautotrophic growth. With regard to their sulfur metabolism, purple bacteria are characterized by a great variability of sulfur substrates used and pathways employed. We will therefore first give an overview about the sulfur oxidation capabilities of the various groups of purple bacteria. Comparison of genome sequence data provides additional insight into the sulfur oxidation pathways. Special attention is given to current knowledge on the biochemical details of the metabolic pathways employed. A variety of enzymes catalyzing sulfur oxidation reactions have been isolated from purple bacteria, and Allochromatium vinosum, a representative of the Chromatiaceae, has been especially well characterized also on a molecular genetic level. Comparative genomics in combination with older biochemical data results in a clear picture of sulfate assimilation and the enzymes involved in purple bacteria.