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Crystal structure of histidyl‐tRNA synthetase from Escherichia coli complexed with histidyl‐adenylate.

The crystal structure at 2.6 A of the histidyl‐tRNA synthetase from Escherichia coli complexed with histidyl‐adenylate has been determined. The enzyme is a homodimer with a molecular weight of 94 kDa and belongs to the class II of aminoacyl‐tRNA synthetases (aaRS). The asymmetric unit is composed of two homodimers. Each monomer consists of two domains. The N‐terminal catalytic core domain contains a six‐stranded antiparallel beta‐sheet sitting on two alpha‐helices, which can be superposed with the catalytic domains of yeast AspRS, and GlyRS and SerRS from Thermus thermophilus with a root‐mean‐square difference on the C alpha atoms of 1.7–1.9 A. The active sites of all four monomers are occupied by histidyl‐adenylate, which apparently forms during crystallization. The 100 residue C‐terminal alpha/beta domain resembles half of a beta‐barrel, and provides an independent domain oriented to contact the anticodon stem and part of the anticodon loop of tRNA(His). The modular domain organization of histidyl‐tRNA synthetase reiterates a repeated theme in aaRS, and its structure should provide insight into the ability of certain aaRS to aminoacylate minihelices and other non‐tRNA molecules.