Beckman D L, Trawick D R, Kranz R G
Department of Biology, Washington University, St. Louis, Missouri 63130.
Genes Dev. 1992 Feb;6(2):268-83. doi: 10.1101/gad.6.2.268.
We report the primary sequence analyses of two loci, hel and ccl, whose gene products are required specifically for the biogenesis of c-type cytochromes in the Gram-negative photosynthetic bacterium Rhodobacter capsulatus. Genetic and molecular analyses show that the hel locus contains at least four genes, helA, helB, helC, and orf52, and the ccl locus contains two genes, ccl1 and ccl2, that are essential for cytochromes c biogenesis. HelA is homologous to a class of proteins called ABC transporters and helA, helB, and helC are proposed to encode an export complex. Cytochrome c2-alkaline phosphatase gene fusions were used to show that apocytochrome c2 synthesis and secretion are not affected by the hel and ccl defects. Ccl1 and Ccl2 possess typical signal sequences to direct them to the periplasm. The periplasmic orientation of Ccl1 was confirmed using a Ccl1-alkaline phosphatase gene fusion. The Ccl1-alkaline phosphatase gene fusion analysis also demonstrated that Ccl1 does not require hel genes for its synthesis and secretion. Ccl1 is homologous to proteins encoded by chloroplast and mitochondrial genes, suggesting analogous functions in these organelles. Taken together, these results support the hypothesis that the hel-encoded proteins are required for the export of heme to the periplasm where it is subsequently ligated to the c-type apocytochromes.
我们报告了两个基因座hel和ccl的一级序列分析，其基因产物是革兰氏阴性光合细菌荚膜红细菌中c型细胞色素生物合成所特需的。遗传和分子分析表明，hel基因座至少包含四个基因，即helA、helB、helC和orf52，而ccl基因座包含两个基因，ccl1和ccl2，它们对于细胞色素c的生物合成至关重要。HelA与一类称为ABC转运蛋白的蛋白质同源，并且推测helA、helB和helC编码一个输出复合体。细胞色素c2-碱性磷酸酶基因融合用于表明脱辅基细胞色素c2的合成和分泌不受hel和ccl缺陷的影响。Ccl1和Ccl2具有典型的信号序列以将它们导向周质。使用Ccl1-碱性磷酸酶基因融合证实了Ccl1的周质定位。Ccl1-碱性磷酸酶基因融合分析还表明，Ccl1的合成和分泌不需要hel基因。Ccl1与叶绿体和线粒体基因编码的蛋白质同源，表明在这些细胞器中具有类似的功能。综上所述，这些结果支持以下假说：hel编码的蛋白质是血红素输出到周质所必需的，在周质中血红素随后与c型脱辅基细胞色素连接。
