A cyclic sulfur compound, identified as cysteine thiolactone by several chemical and enzymatic tests, is formed from cysteine during in vitro tRNACys aminoacylation catalyzed by Escherichia coli cystelnyl-tRNA synthetase. The mechanism of cysteine thiolactone formation involves enzymatic deacylatlon of Cys-tRNACys (k = 0.017 s-1) in which nucleophlllc sulfur of the side chain of cysteine in Cys-tRNACys attacks its carboxyl carbon to yield cysteine thiolactone. Nonenzymatic deacylatlon of Cys-tRNACys (k = 0.0006 s-1) yields cysteine, as expected. Inhibition of enzymatic deacylation of Cys-tRNACys by cysteine and Cys-AMP, but not by ATP, indicates that both synthesis of Cys-tRNACys and cyclization of cysteine to the thiolactone occur in a single active site of the enzyme. The cyclization of cysteine is mechanistically similar to the editing reactions of methlonyl-tRNA synthetase. However, in contrast to methionyl-tRNA synthetase which needs the editing function to reject misactivated homocysteine, cysteinyl-tRNA synthetase is highly selective and is not faced with a problem in rejecting noncognate amlno acids. Despite this, the present day cysteinyl-tRNA synthetase, like methionyl-tRNA synthetase, still retains an editing activity toward the cognate product, the charged tRNA. This function may be a remnant of a chemistry used by an ancestral cysteinyl-tRNA synthetase.
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