Sulfur metabolism in phototrophic bacteria

Abstract

Sulfur is one of the most versatile elements in life due to its reactivity in different oxidation and reduction states. In contrast to the assimilatory provision of sulfur-containing cell constituents that is found in most taxonomic groups, dissimilation is restricted to prokaryotes and serves energy-yielding processes where sulfur compounds are donors or acceptors of electrons. In many anoxygenic phototrophic bacteria, reduced sulfur compounds play a prominent role as electron donors for photosynthetic carbon dioxide fixation. This process is especially characteristic for the green sulfur bacteria (GSB) and the purple sulfur bacteria (PSB). Allochromatium vinosum and Chlorobaculum tepidum, representatives of the PSB and GSB, respectively, are the workhorses for detailed elucidation of sulfur oxidation pathways. Genes identified in these organisms served as the basis of a genome-based survey of the distribution of genes involved in the oxidation of sulfur compounds in other genome-sequenced anoxygenic phototrophs. These analyses show that dissimilatory sulfur metabolism is very complex and built together from various modules encoding different enzymes in the different organisms. Comparative genomics in combination with biochemical data also provide a clear picture of sulfate assimilation in anoxygenic phototrophs.