Kallas T, Spiller S, Malkin R
Division of Molecular Plant Biology, University of California, Berkeley 94720.
J Biol Chem. 1988 Oct 5;263(28):14334-42.
We have isolated and determined the nucleotide and derived protein sequences for the four genes, petCA and BD, which encode the cytochrome b6-f, electron-transfer complex of the filamentous cyanobacterium, Nostoc PCC 7906. The primary structure and cotranscription of the petCA genes encoding the Rieske-FeS (nuclear encoded in plants) and apocytochrome f proteins has been described previously (Kallas, T., Spiller, S., and Malkin, R. (1988) Proc. Natl. Acad. Sci. U.S.A., in press). The petBD genes (645 and 480 protein-coding nucleotides, respectively) for the apocytochrome b6 (24.3 kDa) and subunit-IV (17.5 kDa) proteins comprise a second operon located at least 12 kilobases (kb) from petCA. The Nostoc petBD genes are not closely linked to the psbB gene (encoding the 51-kDa photosystem II polypeptide) and do not contain introns as do the closely related chloroplast genes. DNA probes specific for each of the Nostoc cytochrome-complex genes hybridized to single bands in genomic DNA blots at intensities expected for single copy genes. These data suggest that a single set of cytochrome b6-f proteins function in the different types of membranes found in Nostoc vegetative and heterocyst cells. RNA blot hybridizations identified an 1.8-kb mRNA common to cytochrome b6 and subunit IV, and an intensely hybridizing 0.8-kb mRNA specific to the subunit IV gene probe. The role of the latter RNA is not clear but it may represent a transcript from the opposite strand. The deduced Rieske, apocytochrome f, apocytochrome b6, and subunit IV proteins exhibit 59, 58-63, 84-85, and 79-83% sequence identity with the proteins from chloroplast cytochrome b6-f complexes. The Nostoc proteins show lower but still significant sequences identity with the corresponding proteins of the mitochondrial-type b-c1 complexes. The four probable heme-liganding His residues, and the approximate spacings between them, have been conserved in all of the available cytochrome b6 and b sequences from divergent sources. The Nostoc apocytochrome b6 and subunit IV proteins, as well as the Rieske, appear to be translated and thus inserted into the membrane as mature forms without cleavable presequences. Hydropathy analyses revealed five potential membrane spans in cytochrome b6 and three in the subunit IV protein, consistent with the profiles observed for the chloroplast proteins and the related cytochrome b proteins of cytochrome b-c1 complexes.
我们已经分离并确定了丝状蓝细菌念珠藻Nostoc PCC 7906中编码细胞色素b6-f电子传递复合体的四个基因petCA和BD的核苷酸序列及推导的蛋白质序列。先前已经描述了编码Rieske-FeS（植物中核编码）和脱辅基细胞色素f蛋白的petCA基因的一级结构和共转录情况（卡拉斯，T.，斯皮勒，S.，和马尔金，R.（1988年）《美国国家科学院院刊》，即将发表）。脱辅基细胞色素b6（24.3 kDa）和亚基IV（17.5 kDa）蛋白的petBD基因（分别为645和480个蛋白质编码核苷酸）组成了第二个操纵子，位于距petCA至少12千碱基（kb）处。念珠藻的petBD基因与psbB基因（编码51-kDa光系统II多肽）没有紧密连锁，并且不像密切相关的叶绿体基因那样含有内含子。针对念珠藻细胞色素复合体各基因的DNA探针与基因组DNA印迹中的单一条带杂交，其强度符合单拷贝基因的预期。这些数据表明，一组细胞色素b6-f蛋白在念珠藻营养细胞和异形胞中发现的不同类型膜中发挥作用。RNA印迹杂交鉴定出一种1.8-kb的mRNA是细胞色素b6和亚基IV共有的，以及一种与亚基IV基因探针特异性强烈杂交的0.8-kb mRNA。后一种RNA的作用尚不清楚，但它可能代表来自相反链的转录本。推导的Rieske、脱辅基细胞色素f、脱辅基细胞色素b6和亚基IV蛋白与叶绿体细胞色素b6-f复合体的蛋白分别具有59%、58 - 63%、84 - 85%和79 - 83%的序列同一性。念珠藻的蛋白与线粒体类型b-c1复合体的相应蛋白的序列同一性较低，但仍然显著。四个可能的血红素配位组氨酸残基及其之间的大致间距在来自不同来源的所有可用细胞色素b6和b序列中都得到了保守。念珠藻的脱辅基细胞色素b6和亚基IV蛋白以及Rieske蛋白似乎是以成熟形式翻译并因此插入膜中的，没有可切割的前导序列。亲水性分析揭示细胞色素b6中有五个潜在的跨膜区，亚基IV蛋白中有三个，这与叶绿体蛋白以及细胞色素b-c1复合体中相关细胞色素b蛋白所观察到的图谱一致。
