Wang G, Angermüller S, Klipp W
Lehrstuhl für Genetik, Fakultät für Biologie, Universität Bielefeld, Germany.
J Bacteriol. 1993 May;175(10):3031-42. doi: 10.1128/jb.175.10.3031-3042.1993.
The alternative, heterometal-free nitrogenase of Rhodobacter capsulatus is repressed by traces of molybdenum in the medium. Strains carrying mutations located downstream of nifB copy II were able to express the alternative nitrogenase even in the presence of high molybdate concentrations. DNA sequence analysis of a 5.5-kb fragment of this region revealed six open reading frames, designated modABCD, mopA, and mopB. The gene products of modB and modC are homologous to ChlJ and ChlD of Escherichia coli and represent an integral membrane protein and an ATP-binding protein typical of high-affinity transport systems, respectively. ModA and ModD exhibited no homology to known proteins, but a leader peptide characteristic of proteins cleaved during export to the periplasm is present in ModA, indicating that ModA might be a periplasmic molybdate-binding protein. The MopA and MopB proteins showed a high degree of amino acid sequence homology to each other. Both proteins contained a tandem repeat of a domain encompassing 70 amino acid residues, which had significant sequence similarity to low-molecular-weight molybdenum-pterin-binding proteins from Clostridium pasteurianum. Compared with that for the parental nifHDK deletion strain, the molybdenum concentrations necessary to repress the alternative nitrogenase were increased 4-fold in a modD mutant and 500-fold in modA, modB, and modC mutants. No significant inhibition of the heterometal-free nitrogenase by molybdate was observed for mopA mopB double mutants. The uptake of molybdenum by mod and mop mutants was estimated by measuring the activity of the conventional molybdenum-containing nitrogenase. Molybdenum transport was not affected in a mopA mopB double mutant, whereas strains carrying lesions in the binding-protein-dependent transport system were impaired in molybdenum uptake.
荚膜红细菌的非异金属固氮酶会受到培养基中痕量钼的抑制。携带位于nifB拷贝II下游突变的菌株即使在高浓度钼酸盐存在的情况下也能够表达非异金属固氮酶。对该区域一个5.5 kb片段的DNA序列分析揭示了6个开放阅读框,命名为modABCD、mopA和mopB。ModB和ModC的基因产物与大肠杆菌的ChlJ和ChlD同源,分别代表高亲和力转运系统典型的整合膜蛋白和ATP结合蛋白。ModA和ModD与已知蛋白质无同源性,但ModA中存在在输出到周质期间被切割的蛋白质特有的前导肽,这表明ModA可能是一种周质钼酸盐结合蛋白。MopA和MopB蛋白彼此之间显示出高度的氨基酸序列同源性。两种蛋白都含有一个包含70个氨基酸残基的结构域的串联重复序列,该序列与巴氏梭菌的低分子量钼蝶呤结合蛋白具有显著的序列相似性。与亲本nifHDK缺失菌株相比,抑制非异金属固氮酶所需的钼浓度在modD突变体中增加了4倍,在modA、modB和modC突变体中增加了500倍。对于mopA mopB双突变体,未观察到钼酸盐对非异金属固氮酶有明显抑制作用。通过测量传统含钼固氮酶的活性来估计mod和mop突变体对钼的摄取。mopA mopB双突变体中的钼转运不受影响,而在依赖结合蛋白的转运系统中存在损伤的菌株在钼摄取方面受损。