AbstractHistidyl-tRNA synthetase (HisRS) is responsible for the synthesis of histidyl-transfer RNA, which is essential for the incorporation of histidine into proteins. This amino acid has uniquely moderate basic properties and is an important group in many catalytic functions of enzymes.A compilation of currently known primary structures of HisRS shows that the subunits of these homodimeric enzymes consist of 420–550 amino acid residues. This represents a relatively short chain length among aminoacyl-tRNA synthetases (aaRS), whose peptide chain sizes range from about 300 to 1100 amino acid residues.The crystal structures of HisRS from two organisms and their complexes with histidine, histidyl-adenylate and histidinol with ATP have been solved. HisRS fromThe aminoacylation reaction follows the standard two-step mechanism. HisRS also belongs to the group of aaRS that can rapidly synthesize diadenosine tetraphosphate, a compound that is suspected to be involved in several regulatory mechanisms of cell metabolism. Many analogs of histidine have been tested for their properties as substrates or inhibitors of HisRS, leading to the elucidation of structure-activity relationships concerning configuration, importance of the carboxy and amino group, and the nature of the side chain.HisRS has been found to act as a particularly important antigen in autoimmune diseases such as rheumatic arthritis or myositis. Successful attempts have been made to identify epitopes responsible for the complexation with such auto-antibodies.