Abstract
The Escherichia coli L-cysteine desulfurase IscS mobilizes sulfur from L-cysteine for the synthesis of several biomolecules such as iron-sulfur (FeS) clusters, molybdopterin, thiamin, lipoic acid, biotin, and the thiolation of tRNAs. The sulfur transfer from IscS to various biomolecules is mediated by different interaction partners (e.g. TusA for thiomodification of tRNAs, IscU for FeS cluster biogenesis, and ThiI for thiamine biosynthesis/tRNA thiolation), which bind at different sites of IscS. Transcriptomic and proteomic studies of a ΔtusA strain showed that the expression of genes of the moaABCDE operon coding for proteins involved in molybdenum cofactor biosynthesis is increased under aerobic and anaerobic conditions. Additionally, under anaerobic conditions the expression of genes encoding hydrogenase 3 and several molybdoenzymes such as nitrate reductase were also increased. On the contrary, the activity of all molydoenzymes analyzed was significantly reduced in the ΔtusA mutant. Characterization of the ΔtusA strain under aerobic conditions showed an overall low molybdopterin content and an accumulation of cyclic pyranopterin monophosphate. Under anaerobic conditions the activity of nitrate reductase was reduced by only 50%, showing that TusA is not essential for molybdenum cofactor biosynthesis. We present a model in which we propose that the direction of sulfur transfer for each sulfur-containing biomolecule is regulated by the availability of the interaction partner of IscS. We propose that in the absence of TusA, more IscS is available for FeS cluster biosynthesis and that the overproduction of FeS clusters leads to a modified expression of several genes.
Highlights
The sulfur carrier protein TusA is involved in sulfur transfer to different biomolecules in the cell
The studies presented in this article show that TusA has a role in addition to tRNA thiolation in the cell
We suggest that TusA is involved in sulfur transfer for the synthesis of MPT and, in addition, is involved in the balanced regulation of the availability of IscS to various biomolecules in E. coli
Summary
The sulfur carrier protein TusA is involved in sulfur transfer to different biomolecules in the cell. IscS delivers the sulfur to several sulfur-accepting proteins such as IscU, TusA, and ThiI and, as shown recently, to rhodanese-like proteins such as YnjE [5, 6], thereby providing sulfur for different metabolic pathways like the assembly of FeS cluster, thiamine, biotin, and lipoic acid, tRNA modification, or molybdenum cofactor (Moco) biosynthesis. The TusA protein functions as a sulfur mediator for the synthesis of 2-thiouridine of the modified wobble base 5-methylaminomethyl-2-thiouridine (mnm)5s2U in tRNA [9] It interacts with IscS and stimulates its L-cysteine desulfurase activity. It has been shown that MoeB solely activates the C terminus of MoaD by the formation of an acyl-adenylate, and the L-cysteine desulfurase IscS in its persulfide-bound form has been identified as the primary physiological sulfur-donating enzyme for the generation of thiocarboxylate on MPT synthase [24, 25]. We present a model explaining the different effects of TusA under various growth conditions in E. coli with a focus on Moco biosynthesis
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