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The gene for -glutamylcysteine synthetase from Thiobacillus ferrooxidans has low homology to its Escherichia coli equivalent and is linked to the gene for citrate synthase

Department of Microbiology, University of Cape Town, Rondebosch 7700, South Africa

²Author for correspondence: Douglas Rawlings. Tel: +27 21 650 3261. Fax: +27 21 650 4023. e-mail: doug@molbiol.uct.ac.za

ABSTRACT

The gene for -glutamylcysteine synthetase (gshA) from Thiobacillus ferrooxidans was isolated from a family of cosmids by its ability to complement an Escherichia coli gshA trxA double mutant which was unable to grow on minimal medium lacking glutathione. The predicted sequence of the -glutamylcysteine synthetase was found to have only 18% amino acid sequence identity to the equivalent enzyme from E. coli. In spite of this low sequence homology, concentrations of GSH in a cell extract prepared from the E. coli gshA trxA mutant containing the cloned gene were almost as high as in a cell extract prepared from a wild-type E. coli strain. The gshA gene was found to be physically and transcriptionally linked to the T. ferrooxidans gene for citrate synthase (gltA). The T. ferrooxidans and E. coli citrate synthases shared 37% amino acid sequence identity and the cloned T. ferrooxidans citrate synthase gene was able to complement an E. coli gltA mutant.

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