浑球红假单胞菌GOGAT缺失株中依赖2-酮戊二酸的固氮酶诱导

2-Ketoglutarate-Dependent Induction of Nitrogenase from A GOGAT Mutant of Rhodobacter sphaeroides

2—酮戊二酸加于限量氮培养基中后,引起泽球红假单胞菌谷氨酸合成酶(GOGAT)缺失菌株(asm^-,Nif^-)固氮酶的诱导表达,诱导生成的固氮酶的活性随着培养时间的延长而下降。此时如果加入谷氨酸,固氮酶活性又重新出现,而谷氨酸通常是阻遏GOGAT缺失菌株固氮酶表达的。诱导出的固氮酶与野生菌株固氮酶有相同的调节方式,但前后两次出现的固氮酶活性在氨的敏感性上有差异。此外,在GOGAT缺失菌株内含有较高的谷氨酰胺库,同时谷氨酸胺合成酶(GS)的腺苷酸化状态也较野生型菌株高。

A GOGAT mutant isolated from photosyn-thetic bacterium Rhodobacter sphaeroides showeda Nif minus phenotype, after addition of 2-ke-toglutarate to the nitrogen-limited mutant cul-ture, nitrogenase was induced (Fig. 1). Malate,a common carbon source used in photosynthe-tic bacteria, could not induce the nitrogenasefrom the GOGAT mutant. The nitrogenase ac-tivity decreased during the incubation period,and activity increased when supplementedwith glutamate which usually repressed nitro-genase (Fig. 2). Nitrogenase from two peakshad different sensitivities to ammonia. Theproperties of nitrogenase from peak I resembledthose of nitrogenase expressed in the carbon-starved cells of R. palustris, whereas those frompeak Ⅱ resembled those synthesized in the wildtype of R. sphaeroides (Fgi. 3, 4). The GOGATmutant showed higher content of glutaminewhen analysed with HPLC (Table 1) and a hig-her adenylylation state of glutamine synthe-tase compared with its wild type parent strain.