Isolated, recombinant nucleic acids which encode alanyl-tRNA synthetase (AlaRS) of human origin have been used to make expression constructs and transformed host cells for the production of recombinant human AlaRS. The recombinant enzyme has been purified, and is active in the specific aminoacylation of tRNA by alanine. The isolated, recombinant human AlaRS is also recognized by antibodies made by patients with the particular autoimmune disease known as "antisynthetase syndrome" in which the patients produce antibodies against the human alanyl-tRNA synthetase in their own cells. Thus, the isolated, recombinant enzyme, and antibodies made specifically thereto, can be useful in assays to diagnose and monitor this disease. The essential alanyl-tRNA synthetases of microbes pathogenic in humans can be the targets of inhibitory agents having antimicrobial activity. The human alanyl-tRNA synthetase, isolated and purified, can be used to assess the toxic effect in humans of such an inhibitory agent in various biochemical activity assays. This human enzyme can also be expressed in "tester strains," whose cells rely upon the function of the human alanyl-tRNA synthetase for tRNA.sup.Ala charging. Such tester strains can be used to test for any toxic effects of an antimicrobial agent that specifically interacts with the heterologous human AlaRS gene or gene product.