Sulphur metabolism in Paracoccus denitrificans. Purification, properties and regulation of cysteinyl-and methionyl-tRNA synthetase.

Cysteinyl- and methionyl-tRNA synthetases (EC 6.11.-) were purified 1200- and 1000-fold, respectively, from sonic extracts of Paracoccus denitrificans strain 8944, and kinetics, substrate specificity and regulatory properties were determined using the ATP-PPi exchange reaction. Both enzymes had pH optima of approx. 8 and were inhibited by sulphydryl-group reagents. Cysteinyl-tRNA synthetase catalysed L-selenocysteine- and alpha-aminobutyric acid-dependent ATP-PPi exchange and methionyl-tRNA synthetase catalysed L-homocysteine-, L-selenomethionine- and norleucine-dependent ATP-PPi exchange. Both enzymes were inhibited by O-acetylserine. Cysteinyl-tRNA synthetase activity was stimulated by methionine and methionyl-tRNA synthetase activity was stimulated by sulphide, cysteine, and cysteic acid.