Probably involved in nitrogen metabolism via ammonium assimilation. Catalyzes the ATP-dependent biosynthesis of glutamine from glutamate and ammonia. ATP + L-glutamate + NH4(+) = ADP + H(+) + L-glutamine + phosphate Binds 2 Mg(2+) or Mn(2+) ions per subunit. Strongly inhibited by glycine and L-alanine. AMP at 10 mM displays a very weak inhibitory effect. The activity of this enzyme is not controlled by adenylation. Optimum pH is between 7 and 7.5. Optimum temperature is 90 degrees Celsius. In the absence of magnesium or manganese ions about 50% of the activity is lost within 100 minutes at 78 degrees Celsius, whereas more than 95% of the activity is retained in presence of 4 mM manganese ions. Magnesium ions are a less effective. Oligomer of 12 subunits arranged in the form of two hexagons. GlnA of S.acidocaldarius is unique among the archeal glutamine synthetase since the regulatory properties suggest a position within the GS I-alpha subgroup. However, the sequence shows more pronounced similarities to the GS I-beta subgroup. Belongs to the glutamine synthetase family.