In this study, we have substituted serine-43 by cysteine in the recombinant citrate synthase from a moderately thermophilic Archaeon Thermoplasma acidophilum, for site-specific attachment of labels and have investigated the effects of this mutation on the biochemical properties and thermal stability of the enzyme. Both wild-type and the mutant enzymes were purified to homogenity using affinity chromatography on Matrex Gel Red A. The mutant Thermoplasma citrate synthase is very similar to wild-type citrate synthase in its substrate and co-factor specificities, pH profile and thermal stability. The mutation, however, has decreased the enzyme activity. The newly introduced reactive sulphydryl group could be easily modified by DTNB and labelled with 4-chloro-7-sulphobenzofuran, without loss of any activity. (C) 1996 Academic Press, Inc.