Department of Biology, University of Copenhagen, Ole Maaløes Vej 5, 2200 Copenhagen N, Denmark.
Microbiology (Reading). 2011 Apr;157(Pt 4):1229-1239. doi: 10.1099/mic.0.044669-0. Epub 2011 Jan 13.
Green sulfur bacteria (GSB) oxidize sulfide and thiosulfate to sulfate, with extracellular globules of elemental sulfur as an intermediate. Here we investigated which genes are involved in the formation and consumption of these sulfur globules in the green sulfur bacterium Chlorobaculum tepidum. We show that sulfur globule oxidation is strictly dependent on the dissimilatory sulfite reductase (DSR) system. Deletion of dsrM/CT2244 or dsrT/CT2245, or the two dsrCABL clusters (CT0851-CT0854, CT2247-2250), abolished sulfur globule oxidation and prevented formation of sulfate from sulfide, whereas deletion of dsrU/CT2246 had no effect. The DSR system also seems to be involved in the formation of thiosulfate, because thiosulfate was released from wild-type cells during sulfide oxidation, but not from the dsr mutants. The dsr mutants incapable of complete substrate oxidation oxidized sulfide and thiosulfate about twice as fast as the wild-type, while having only slightly lower growth rates (70-80 % of wild-type). The increased oxidation rates seem to compensate for the incomplete substrate oxidation to satisfy the requirement for reducing equivalents during growth. A mutant in which two sulfide : quinone oxidoreductases (sqrD/CT0117 and sqrF/CT1087) were deleted exhibited a decreased sulfide oxidation rate (~50 % of wild-type), yet formation and consumption of sulfur globules were not affected. The observation that mutants lacking the DSR system maintain efficient growth suggests that the DSR system is dispensable in environments with sufficiently high sulfide concentrations. Thus, the DSR system in GSB may have been acquired by horizontal gene transfer as a response to a need for enhanced substrate utilization in sulfide-limiting habitats.
绿色硫细菌（GSB）将硫化物和硫代硫酸盐氧化为硫酸盐，同时形成元素硫的胞外球作为中间产物。在这里，我们研究了绿色硫细菌绿屈挠菌中形成和消耗这些硫球的相关基因。我们发现硫球的氧化过程严格依赖于异化亚硫酸盐还原酶（DSR）系统。dsrM/CT2244 或 dsrT/CT2245 或两个 dsrCABL 簇（CT0851-CT0854、CT2247-2250）的缺失，会导致硫球氧化作用的完全丧失和硫化物到硫酸盐的转化受阻，而 dsrU/CT2246 的缺失则没有影响。DSR 系统似乎也参与了硫代硫酸盐的形成，因为在硫化物氧化过程中，野生型细胞会释放出硫代硫酸盐，但 dsr 突变体则不会。不能完全氧化底物的 dsr 突变体氧化硫化物和硫代硫酸盐的速度比野生型快两倍左右，而生长速度则略低（野生型的 70-80%）。这种增加的氧化速率似乎可以补偿不完全的底物氧化，以满足生长过程中对还原当量的需求。两个硫代硫酸盐：醌氧化还原酶（sqrD/CT0117 和 sqrF/CT1087）缺失的突变体的硫化物氧化速率下降（约为野生型的 50%），但硫球的形成和消耗没有受到影响。缺乏 DSR 系统的突变体仍能维持高效的生长，这表明在有足够高的硫化物浓度的环境中，DSR 系统是可有可无的。因此，GSB 中的 DSR 系统可能是通过水平基因转移获得的，以应对在硫化物限制的生境中需要增强底物利用的需求。
