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 缺乏高亲和力同化半胱氨酸导入蛋白。


微生物吸收硫的最直接途径是输入半胱氨酸。然而，在氨基酸中，半胱氨酸没有明确的输入者。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，它主要通过专门用于支链氨基酸的 LIV ATP 驱动系统来实现。该系统对半胱氨酸的亲和力远低于对 Leu、Ile 和 Val 的亲和力，因此当它们存在时，半胱氨酸被排除在外。因此，这种运输不太可能与自然环境相关。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同化的高亲和力输入蛋白。肠道细菌不含有在空肠弯曲杆菌中鉴定出的推定的半胱氨酸输入源。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导入胱氨酸（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胱氨酸，这是大肠杆菌输入的。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，这种替代方案几乎同样经济，并且避免了过量半胱氨酸进入细胞时可能产生的毒性作用。