The glutamine synthetase from the hyperthermoacidophilic crenarcheon Sulfolobus acidocaldarius: Isolation, characterization and sequencing of the gene

The glutamine synthetase (EC 6.3.1.2) from the hyperthermoacidophilic crenarcheon Sulfolobus acidocaldarius (DSM 639) was purified to homogeneity, characterized and the glnA gene isolated and sequenced. The amount of enzyme present in the cytosolic fraction from Sulfolobus cells showed a strong variation depending on the carbon and nitrogen sources in the growth medium. The enzyme was found to be a dodecameric protein composed of identical subunits of 52 kDa. It was stable at 78°C in the presence of Mn²⁺ ions. The catalytic activity was regulated solely by feed-back inhibition through L-alanine and glycine and not by adenylylation. No evidence for the presence of isoenzymes was found. Sequence comparison showed that the Sulfolobus protein is most closely related to the glutamine synthetases of the I-β type despite its regulatory properties and the finding that the known euryarcheal glutamine synthetase sequences belong to the I-α subgroup of these enzymes. Our phylogenetic analysis suggests that the gene duplication leading to the development of the I-α and I-β enzymes preceded the separation of the archea and the bacteria.