Escherichia coli K-12 Lacks a High-Affinity Assimilatory Cysteine Importer

Abstract

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. IMPORTANCE This investigation discovered that Escherichia coli lacks a transporter dedicated to the assimilation of cysteine, an outcome that is in striking contrast to the many transporters devoted to the other 19 amino acids. We ascribe the lack of a high-affinity cysteine importer to two considerations. First, the chemical reactivity of this amino acid is unique, and its poorly controlled import can have adverse consequences for the cell. Second, our analysis suggests that the economics of biosynthesis depend sharply upon whether the cell is respiring or fermenting. In the anoxic habitats in which cysteine might be found, the value of import versus biosynthesis is strongly reduced compared to that in oxic habitats. These studies may explain why bacteria choose to synthesize rather than to import other useful biomolecules as well.

Cysteine dependence in Lactobacillus iners constitutes a novel therapeutic target to modify the vaginal microbiota

“…We hypothesized that L. iners ’s restricted ability to use exogenous Cys sources might reflect a limited repertoire of transport mechanisms for Cys and Cys-containing molecules in comparison to other lactobacilli. Cys transporters are not well-characterized in bacteria compared to transporters for other amino acids 47 . Uptake of Cys and Cys-containing molecules is described as occurring through multiple known or putative mechanisms.…”

“…Uptake of Cys and Cys-containing molecules is described as occurring through multiple known or putative mechanisms. These include the well-characterized cystine transport systems TcyABC, TcyJKLMN, and TcyP 48 , a putative Cys ABC transporter associated with the Cys-binding protein CjaA 47, 49 , and the heterodimeric ABC transporter CydDC encoded by the redox-regulating locus cydABCD 50 , a conserved system that exports both Cys and GSH to the periplasm in E. coli 50 and is proposed to perform glutathione uptake in lactobacilli 51 . Analysis of our expanded Lactobacillus genome catalogs annotated using eggNOG revealed that genomes of all non- iners species encoded cydABCD , while L. crispatus , L. vaginalis , and some L. gasseri encoded one or more predicted cjaA orthologs and L. jensenii , L. gasseri , and L. vaginalis each encoded one or more cystine transport systems (Fig.…”

“…4b and Supplementary Table 4). Low-affinity Cys uptake can also occur via the branched-chain amino acid importer LivJ/LivKHMGF 47 , but no species possessed livKHMGF orthologs except 2 L. iners MAGs (out of 327 genomes), nor did any species possess the full GsiABCD glutathione transport system (Extended Data Fig 6a and Supplementary Table 4) 52 . The findings were supported by results of BLAST 42 searches against the genome collections using gene sequences of interest from related species (Supplementary File 1).…”

“…A striking aspect of our findings is the paucity of transport mechanisms for Cys and Cys-containing molecules in L. iners compared to other FGT lactobacilli. Unlike other amino acids, transporters for Cys are not well characterized in bacteria due to technical challenges inherent to redox chemistry that complicate Cys quantification and efforts to distinguish between transport of reduced Cys and transport of its oxidized counterpart, cystine 47 . The relatively few functional studies of putative bacterial Cys transporters have focused predominantly on Gram-negative bacteria from the phylum Proteobacteria rather than lactobacilli or related Gram-positive species 47, 49 .…”

“…Uptake of Cys and Cys-containing molecules is described as occurring through multiple known or putative mechanisms. These include the well-characterized cystine transport systems TcyABC, TcyJKLMN, and TcyP 48 , a putative Cys ABC transporter associated with the Cys-binding protein CjaA 47,49 , and the heterodimeric ABC transporter CydDC encoded by the redox-regulating locus cydABCD 50 , a conserved system that exports both Cys and . CC-BY 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.…”