GLUTAMINE-SYNTHETASE REGULATION BY ENERGY-CHARGE IN SUNFLOWER ROOTS

Energy charge [(ATP) + 1/2 (ADP)]/[(ATP) + (ADP) + (AMP)] and glutamine synthetase [EC-6.3.1.2] activity (transferase reaction) of roots increase in a near congruent manner when decotyledonized sunflower plants (Helianthus annuus L. cv. ''Mammoth Russian'') are grown in NO3 for 9 days. Replacement of NO3 with NH4 for the final 2 days leads to a higher energy charge and increased enzyme activity. Similar correlations occur when NO3 plants are placed on a zero N regimen and when they are subjected to continuous darkness. A rank order correlation of 0.72 is obtained for all data. Control concepts such as adenylylation-deadenylylation and NH4 inhibition of enzyme synthesis are not supported by the data. Energy charge-enzyme activity plots support the view that glutamine synthetase of sunflower roots is subject to control by end products of glutamine metabolism. Alanine appears to exert a modulating effect on the regulation of glutamine synthetase by energy charge.