谷氨酰胺合成酶在固氮鱼腥藻固氮调节中的作用

THE ROLE OF GLUTAMINE SYNTHETASE IN REGULAING NITROGENASE ACTIVITY IN ANABAENA AZOTICA

在MSX(methlonine sulfoximine,谷氨酰胺合成酶的不可逆抑制剂)存在下,固氮鱼腥藻(Anabaena azotica)所分泌的氨量和谷氨酰胺合成酶(GS)活力有较好的负相关性,证明谷氨酰胺合成酶-谷氨酸合成酶(GS-GOGAT)是固氮鱼腥藻氨同化的重要途径。在蛋白质合成受到氯霉素拟制时,NH_4^+对固氮酶的失活是迅速的,同时GS活力有较大下降,表明NH_4^+的调控酶的失活或降解。在氮固定条件下,固氮酶活力半衰期小于4小时,GS活力半衰期大于10小时,则GS并不是固氮酶的正调节因子。NH_4^+和谷氨酰胺(gln)对固氮酶的失活作用随它的浓度增加而提高,但GS并没有这种相关性,低浓度NH_4^+(0.1—0.5mmol/L·NH_4Cl)对GS活力没有抑制作用,高浓度gln(1.0—2.0mmol/L)也没有抑制GS活力,说明GS并不直接调控固氮酶。MSX能消除NH_4^+和gln对固氮酶的抑制作用,并与藻龄有关。

The addition of L-methionine-DL-sulpheximine (MSX) to the growing culture of Anabaena azotica resultes in releasing newly fixed NH_3 into the medium and inhibits glutamine synthetase activity, showing a negative correlation between GS activity and ammonia production. It indicates that glutamine synthetase-glutamate synthetase pathway is essential to NH_3 assimilation in A. azotica. When protein synthesis was inhibited by chloramphenicol, the addition of NH_4^+ decreased nitrogenase activity and glutamine synthetase activity, indicating inactivity or degradation of the NH_4^+-mediated enzyme. Under nitrogen-fixing condition, the halflife of the activity of nitrogenase was shorter then 4h, Whereas that of glutamine synthetase was longer than 10h. This suggests that glutamine synthetase may not be a positive regulator of nitrogenase. The inactivation effect of NH_4^+ or glutamine on nitrogenase increases with their concentration. However, there is no such relationship for GS. A low concentration of NH_4^+ (0.1—0.5 mmol/L. NH_4Cl) and even a high concentration of gln (1.0—2.0 mol/L.) does not inhibit Gs activity. Thus, it suggests that nitrogenase is not directly regulated by GS. Furthemore, the depression of nitrogenase by NH_4^+ and gln can be eliminated by MSX, of which the function is in relation to the age of the algal culture.