In enzymology, a cysteine—tRNA ligase (EC 6.1.1.16) is an enzyme that catalyzes the chemical reaction:

ATP + L-cysteine + tRNACys AMP + diphosphate + L-cysteinyl-tRNACys

The three substrates of this enzyme are ATP, L-cysteine, and tRNA(Cys), whereas its three products are AMP, diphosphate, and L-cysteinyl-tRNA(Cys).

This enzyme belongs to the family of ligases, specifically those forming carbon-oxygen bonds in aminoacyl-tRNA and related compounds. The systematic name of this enzyme class is L-cysteine:tRNACys ligase (AMP-forming). Other names in common use include cysteinyl-tRNA synthetase, cysteinyl-transferRNA synthetase, cysteinyl-transfer ribonucleate synthetase, and cysteine translase. This enzyme participates in cysteine metabolism and aminoacyl-tRNA biosynthesis.

As of late 2007, three structures have been solved for this class of enzymes, with PDB accession codes 1LI5, 1LI7, and 1U0B.

• Mccorquodale DJ (December 1964). "The separation and partial purification of aminoacyl-RNA synthetases from Escherichia coli". Biochimica et Biophysica Acta (BBA) - Specialized Section on Nucleic Acids and Related Subjects. 91 (4). Elsevier: 541–8. doi:10.1016/0926-6550(64)90001-5. ISSN 0926-6550. PMID 14262440.