The most direct route by which microbes might assimilate sulfur would be by importing cysteine. However, alone among the amino acids, cysteine does not have well-characterized importers. We determined that Escherichia coli can rapidly import cysteine, but in our experiments, it did so primarily through the LIV ATP-driven system that is dedicated to branched-chain amino acids. The affinity of this system for cysteine is far lower than for Leu, Ile, and Val, and so in their presence, cysteine is excluded. Thus, this transport is unlikely to be relevant in natural environments. Growth studies, transcriptomics, and transport assays failed to detect any high-affinity importer that is dedicated to cysteine assimilation. Enteric bacteria do not contain the putative cysteine importer that was identified in Campylobacter jejuni. This situation is surprising, because E. coli deploys ion- and/or ATP-driven transporters that import cystine, the oxidized form of cysteine, with high affinity and specificity. We conjecture that in oxic environments, molecular oxygen oxidizes environmental cysteine to cystine, which E. coli imports. In anoxic environments where cysteine is stable, the cell chooses to assimilate hydrogen sulfide instead. Calculations suggest that this alternative is almost as economical, and it avoids the toxic effects that can result when excess cysteine enters the cell.

中文翻译：

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大肠杆菌K-12缺乏高亲和力的同化半胱氨酸进口商。

微生物吸收硫的最直接途径是输入半胱氨酸。但是，仅在氨基酸中，半胱氨酸就没有特征明确的进口商。我们确定大肠杆菌可以快速导入半胱氨酸，但在我们的实验中，它主要是通过LIV ATP驱动的系统完成的，该系统专用于支链氨基酸。该系统对半胱氨酸的亲和力远低于对Leu，Ile和Val的亲和力，因此在它们存在的情况下，排除了半胱氨酸。因此，这种运输不太可能与自然环境有关。生长研究，转录组学和转运分析未能检测到任何专门用于半胱氨酸吸收的高亲和力进口商。肠细菌不包含在空肠弯曲菌。这种情况令人惊讶，因为大肠杆菌部署了离子和/或ATP驱动的转运蛋白，以高亲和力和特异性进口胱氨酸（半胱氨酸的氧化形式）。我们推测，在有氧环境中，分子氧将环境中的半胱氨酸氧化为大肠杆菌进口的胱氨酸。在半胱氨酸稳定的缺氧环境中，细胞会选择吸收硫化氢。计算表明，这种选择几乎是经济的，并且避免了过量的半胱氨酸进入细胞时可能产生的毒性作用。