Weidenhaupt M, Rossi P, Beck C, Fischer H M, Hennecke H
Mikrobiologisches Institut, Eidgenössische Technische Hochschule, ETH-Zentrum, Schmelzbergstrasse 7, CH-8092 Zürich, Switzerland.
Arch Microbiol. 1996 Mar;165(3):169-78. doi: 10.1007/BF01692858.
A group of four co-regulated genes (fixA, fixB, fixC, fixX) essential for symbiotic nitrogen fixation has been described in several rhizobial species, including Bradyrhizobium japonicum. The complete nucleotide sequence of the B. japonicum fixA, fixB and fixC, genes is reported here. The derived amino acid sequences confirmed the previously noted sequence similarity between FixA and the beta-subunit and between FixB and the alpha-subunit of mammalian and Paracoccus denitrificans electron transfer flavoproteins (ETF). Since the classical role of ETF is in beta-oxidation of fatty acids, a process unrelated to nitrogen fixation, we rationalized that B. japonicum ought to contain bona fide etf genes in addition to the etf-like genes fixA and fixB. Therefore, we identified, cloned, sequenced, and transcriptionally analyzed the B. japonicum etfSL genes encoding the beta- and alpha-subunits of ETF. The etfSL genes, but not the fix genes, are transcribed in aerobically grown cells. An amino acid sequence comparison between all available ETFs and ETF-like proteins revealed the existence of two distinguishable subfamilies. Group I comprises housekeeping ETFs that link acyl-CoA dehydrogenase reactions with the respiratory chain, such as in the fatty acid degradation pathway. B. japonicum EtfS and EtfL clearly belong to this group. Group II contains ETF-like proteins that are synthesized only under certain specific growth conditions and receive electrons from the oxidation of specific substrates. The products of the anaerobically induced fixA and fixB genes of B. japonicum are members of that group. B. japonicum is the first example of an organism in which genes for proteins of both groups I and II of the ETF family have been identified.
在包括慢生根瘤菌在内的几种根瘤菌中，已经发现了一组对共生固氮至关重要的共调控基因（fixA、fixB、fixC、fixX）。本文报道了慢生根瘤菌fixA、fixB和fixC基因的完整核苷酸序列。推导的氨基酸序列证实了之前所指出的，FixA与哺乳动物和反硝化副球菌电子传递黄素蛋白（ETF）的β亚基之间，以及FixB与ETF的α亚基之间的序列相似性。由于ETF的经典作用是参与脂肪酸的β氧化，而这一过程与固氮无关，我们推测慢生根瘤菌除了有类ETF基因fixA和fixB外，应该还含有真正的ETF基因。因此，我们鉴定、克隆、测序并转录分析了编码ETF的β亚基和α亚基的慢生根瘤菌etfSL基因。etfSL基因，而非fix基因，在有氧生长的细胞中被转录。对所有可用的ETF和类ETF蛋白进行的氨基酸序列比较显示，存在两个可区分的亚家族。第一组包括管家ETF，它们将酰基辅酶A脱氢酶反应与呼吸链相连，如在脂肪酸降解途径中。慢生根瘤菌的EtfS和EtfL显然属于这一组。第二组包含仅在特定生长条件下合成并从特定底物氧化中接收电子的类ETF蛋白。慢生根瘤菌厌氧诱导的fixA和fixB基因的产物属于该组。慢生根瘤菌是首个被鉴定出同时含有ETF家族第一组和第二组蛋白质基因的生物体实例。
