This study examined glutamine synthetase II (GSII) from Rhizobium leguminosarum biovar viceae, a bacterium involved in nitrogen-fixing symbiosis with legume plants. Researchers purified GSII to homogeneity and characterized its structure, finding it comprises eight identical 42,000 Da subunits arranged as an octamer that can reversibly dissociate into tetramers. The amino-terminal sequence showed high similarity to GSII from related rhizobia. Using a highly specific antiserum raised against pure GSII, researchers tracked changes in enzyme activity when ammonium chloride was added to cultures previously grown on glutamate. They discovered that transferase activity decreased much faster than the immunoreactive protein itself, indicating post-translational modification rather than protein degradation. Biosynthetic activity measured by different assays showed differential responses: reactions with hydroxylamine decreased rapidly, while ADP formation paralleled protein loss. These findings suggest ammonia availability triggers a chemical modification of GSII that selectively reduces its transferase activity while preserving other catalytic functions, potentially relevant to nitrogen regulation in symbiotic root nodules.

Key findings

• GSII from R. leguminosarum is an octamer of 42 kDa subunits that reversibly dissociates into tetramers under specific conditions
• Transferase activity is rapidly lost upon ammonium chloride treatment while the GSII protein remains intact and immunologically detectable
• Post-translational modification of GSII selectively reduces transferase activity while partially preserving biosynthetic activity measured by ADP formation
• The modification appears to affect glutamate activation on the enzyme surface, making it less susceptible to hydroxylamine reaction
• GSII from R. leguminosarum shares high amino-terminal sequence similarity with GSII from other rhizobia, suggesting conserved functional domains