Bott M, Thöny-Meyer L, Loferer H, Rossbach S, Tully R E, Keister D, Appleby C A, Hennecke H
Mikrobiologisches Institut, Eidgenössische Technische Hochschule, Zürich, Switzerland.
J Bacteriol. 1995 Apr;177(8):2214-7. doi: 10.1128/jb.177.8.2214-2217.1995.
Bradyrhizobium japonicum possesses three soluble c-type cytochromes, c550, c552, and c555. The genes for cytochromes c552 (cycB) and c555 (cycC) were characterized previously. Here we report the cloning, sequencing, and mutational analysis of the cytochrome c550 gene (cycA). A B. japonicum mutant with an insertion in cycA failed to synthesize a 12-kDa c-type cytochrome. This protein was detectable in the cycA mutant complemented with cloned cycA, which proves that it is the cycA gene product. The cycA mutant, a cycB-cycC double mutant, and a cycA-cycB-cycC triple mutant elicited N2-fixing root nodules on soybean (Nod+ Fix+ phenotype); hence, none of these three cytochromes c is essential for respiration supporting symbiotic N2 fixation. However, cytochrome c550, in contrast to cytochromes c552 and c555, was shown to be essential for anaerobic growth of B. japonicum, using nitrate as the terminal electron acceptor.
慢生根瘤菌拥有三种可溶性c型细胞色素，即c550、c552和c555。先前已对细胞色素c552（cycB）和c555（cycC）的基因进行了表征。在此，我们报告细胞色素c550基因（cycA）的克隆、测序及突变分析。cycA基因发生插入突变的慢生根瘤菌突变体无法合成一种12 kDa的c型细胞色素。在导入克隆cycA基因而得到互补的cycA突变体中可检测到这种蛋白质，这证明它就是cycA基因的产物。cycA突变体、cycB - cycC双突变体以及cycA - cycB - cycC三突变体均可在大豆上诱导出固氮根瘤（固氮结瘤表型）；因此，这三种细胞色素c对于支持共生固氮的呼吸作用均非必需。然而，与细胞色素c552和c555不同，细胞色素c550对于以硝酸盐作为末端电子受体的慢生根瘤菌厌氧生长而言是必需的。
