A novel candidate for the true fructose-1,6-bisphosphatase in archaea.

Fructose-1,6-bisphosphatase (FBPase) is one of the key enzymes of the gluconeogenic pathway. Although enzyme activity had been detected in Archaea, the corresponding gene had not been identified until a presumable inositol monophosphatase gene from Methanococcus jannaschii was found to encode a protein with both inositol monophosphatase and FBPase activities. Here we display that a gene from the hyperthermophilic archaeon, Thermococcus kodakaraensis KOD1, which does not correspond to the inositol monophosphatase gene from M. jannaschii, displays high FBPase activity. The FBPase from strain KOD1 was partially purified, its N-terminal amino acid sequence was determined, and the gene (Tk-fbp) was cloned. Tk-fbp encoded a protein of 375 amino acid residues with a molecular mass of 41,658 Da. The recombinant Tk-Fbp was purified and characterized. Tk-Fbp catalyzed the conversion of fructose 1,6-bisphosphate to fructose 6-phosphate following Michaelis-Menten kinetics with a K(m) value of 100 microm toward fructose 1,6-bisphosphate, and a k(cat) value of 17 s(-1) subunit(-1) at 95 degrees C. Unlike the inositol monophosphatase from M. jannaschii, Tk-Fbp displayed strict substrate specificity for fructose 1,6-bisphosphate. Activity was enhanced by Mg(2+) and dithioerythritol, and was slightly inhibited by fructose 2,6-bisphosphate. AMP did not inhibit the enzyme activity. We examined whether expression of Tk-fbp was regulated at the transcription level. High levels of Tk-fbp transcripts were detected in cells grown on pyruvate or amino acids, whereas no transcription was detected when starch was present in the medium. Orthologue genes corresponding to Tk-fbp with high similarity are present in all the complete genome sequences of thermophilic Archaea, including M. jannaschii, Pyrococcus furiosus, Sulfolobus solfataricus, and Archaeoglobus fulgidus, but are yet to be assigned any function. Taking into account the high FBPase activity of the protein, the strict substrate specificity, and its sugar-repressed gene expression, we propose that Tk-Fbp may represent the bona fide FBPase in Archaea.

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