肺炎克氏杆菌nifA基因在巴西固氮螺菌固氮基因表达的铵调节中的作用

固氯螺菌(Azospirillum)是一类仅在限铵和微好氧条件下固氮的微生物，它可与许多禾本科作物联合共生⑴，具有较大的应用潜力。铵作为固氮作用的调节信号，在固氮螺菌的实际应用中是首要的限制因素。在固氯螺菌中，铵不但具有与肺炎克氏杆菌(Klebsiella pneumoniae)相似的阻遏固氮酶合成的作用，而且还对已合成的固氮酶进行活性调节⑵。研究表明，其固氮酶翻译后活性调节的机制类似于深红红螺菌(Rhodospirillum rubrum)⑶，即在有铵条件下其固氮酶铁蛋白的一个亚基被共价修饰而丧失活性，这一过程是可逆的。由于铵在固氮螺菌中双水平地调节固氮作用，使得在野生菌株中研究其固氮基因表达水平上的调节较为困难。Zhang等⑷利用区域定位诱变技术获得了巴西固氮螺菌Sp7（A．Brasilense Sp7)的draT-突变株，在该突变株中铵不再影响固氮酶的活性，这为其固氮基因表达调节的研究提供了一个良好的材料。本文将组成型表达的肺炎克氏杆菌nifA基围引入该突变株中，通过分析讨论铵对巴西固氮螺菌固氮基固表达的调节作用方式。

Effect of Klebsiella pneumoniae nif A on the Regulation of nif Gene Expression by Ammonia in Azospirillum brasilense

Plasmid pCK3, which carried a constitutive nifA gene of Klebsiella pneumoni-ae, was transferred into a dra T- mutant strain of Azospirillum brasilense. It was found that pCK3 could not derepress nif gene expression of A. brasilense under high concentration of ammonia. Combined with the result of the phenotypes of the kanamysin resistant gene from pCK3 in A. brasilense under different incubated conditions, it demonstrated that NifA activity was inhibited by ammonia. We suppose that there is a negative regulator in A. brasilense, although not existing in K. pneumoniae, wihich has effect on NifA activity by ammonia.