The Nondiscriminating Aspartyl-tRNA Synthetase from Helicobacter pylori: Anticodon-Binding Domain Mutations That Impact tRNA Specificity and Heterologous Toxicity

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Abstract
Divergent tRNA substrate recognition patterns distinguish the two distinct forms of aspartyl-tRNA synthetase (AspRS) that exist in different bacteria. In some cases, a canonical, discriminating AspRS (D-AspRS) specifically generates Asp-tRNAAsp and usually coexists with asparaginyl-tRNA synthetase (AsnRS). In other bacteria, particularly those that lack AsnRS, AspRS is nondiscriminating (ND-AspRS) and generates both Asp-tRNAAsp and the noncanonical, misacylated Asp-tRNAAsn; this misacylated tRNA is subsequently repaired by the glutamine-dependent Asp-tRNAAsn/Glu-tRNAGln amidotransferase (Asp/Glu-Adt). The molecular features that distinguish the closely related bacterial D-AspRS and ND-AspRS are not well-understood. Here, we report the first characterization of the ND-AspRS from the human pathogen Helicobacter pylori (H. pylori or Hp). This enzyme is toxic when heterologously overexpressed in Escherichia coli. This toxicity is rescued upon coexpression of the Hp Asp/Glu-Adt, indicating that Hp Asp/Glu-Adt can utilize E. coli Asp-tRNAAsn as a substrate. Finally, mutations in the anticodon-binding domain of Hp ND-AspRS reduce this enzyme's ability to misacylate tRNAAsn, in a manner that correlates with the toxicity of the enzyme in E. coli.