Bishop P E, Jarlenski D M, Hetherington D R
Proc Natl Acad Sci U S A. 1980 Dec;77(12):7342-6. doi: 10.1073/pnas.77.12.7342.
Two Azotobacter vinelandii strains capable of growing on N2(Nif+) were isolated from two different mutant strains that lacked dinitrogenase activity (Nif-). Extracts of N2-grown cells of the two Nif+ strains lacked significant amounts of the "conventional" dinitrogenase protein subunits, as determined by two-dimensional gel electrophoresis. Instead, the extracts contained at least four new proteins that appeared to be ammonia-repressible (i.e., they were not detected in extracts of ammonia-grown cells). Based on the results of genetic backcrosses, the two Nif+ strains were shown to be pseudorevertants. Both Nif+ pseudorevertant strains were able to grow in N-free media lacking molybdenum but containing tungsten (conditions that prevented growth of the wild-type strain). The four new proteins were observed in extracts of N2-fixing cells of the Nif+ pseudorevertants regardless of whether the cells were grown in the presence of molybdenum-starved wild-type A. vinelandii cells grown under N2-fixing conditions. Under conditions of molybdenum deprivation, Nif- mutant strains of several different phenotypic classes underwent phenotypic reversal to Nif+, as shown by their ability to incorporate 15N2 and to grow in N-free media. These results provide evidence that A. vinelandii possesses an alternative N2-fixation system that is expressed during conditions of molybdenum deficiency.
从两种缺乏固氮酶活性（Nif-）的不同突变菌株中分离出了两种能够在N₂上生长的棕色固氮菌菌株（Nif+）。通过二维凝胶电泳测定，这两种Nif+菌株在N₂上生长的细胞提取物中缺乏大量“常规”的固氮酶蛋白质亚基。相反，提取物中含有至少四种新蛋白质，这些蛋白质似乎可被氨抑制（即，在氨生长细胞的提取物中未检测到）。基于遗传回交的结果，这两种Nif+菌株被证明是假回复体。两种Nif+假回复体菌株都能够在不含钼但含钨的无氮培养基中生长（这些条件会阻止野生型菌株生长）。无论细胞是在钼缺乏的野生型棕色固氮菌细胞存在下生长，还是在N₂固定条件下生长，在Nif+假回复体的N₂固定细胞提取物中都观察到了这四种新蛋白质。在钼缺乏的条件下，几种不同表型类别的Nif-突变菌株发生表型逆转成为Nif+，这通过它们结合¹⁵N₂的能力以及在无氮培养基中生长的能力得以证明。这些结果提供了证据，表明棕色固氮菌拥有一种在钼缺乏条件下表达的替代N₂固定系统。
