Cysteinyl-tRNA Synthetase from Bacillus stearothermophilus. A Structural and Functional Monomer

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Abstract
A procedure is described for the purification of cysteinyl-tRNA synthetase as a side product of a multi-enzyme isolation from B. stearothermophilus. The native and denatured enzymes both have a MW of 54,000 by gel filtration and sodium dodecyl sulfate/polyacrylamide gel electrophoresis, respectively. Fingerprinting and peptide counting indicate that the polypeptide chain has a nonrepeating primary structure. The enzyme has only 1 binding site for each of its substrates (cysteine, ATP and tRNACys) as judged by equilibrium dialysis, active-site titration and fluorescence quenching. No evidence for the dimerization of the enzyme in the presence of these substrates was found. Cysteinyl-tRNA synthetase, which is the smallest aminoacyl-tRNA synthetase yet described, is both structurally and functionally monomeric.