Formanová N, Brown G G
Department of Biology, McGill University, Montreal, Quebec, Canada.
Plant Mol Biol. 1997 Jun;34(3):383-92. doi: 10.1023/a:1005818631401.
RNA b is the most abundant member of a family of autonomously replicating single- and double-stranded RNA plasmids found in maize mitochondria. The extent to which this molecule is associated with proteins was investigated by rate zonal and CsCl equilibrium density gradient centrifugation of clarified lysates of S cytoplasm maize mitochondria. A soluble complex of RNA b, responsible for synthesis of the more abundant (+) RNA b strand in mitochondrial lysates, was identified. The complex had a buoyant density of 1.49 g/cm3, indicating a substantial non-nucleic acids content. The sedimentation coefficient of the complex, however, was only slightly larger than that of deproteinized RNA b. Synthesis of RNA b as well as the larger RNA plasmid, RNA a, was resistant to heparin, suggesting that, for both RNAs, preformed complexes between an RNA template and an RNA-dependent RNA polymerase capable of elongating in vivo preinitiated RNA plasmid strands, were present in the lysate. Only a small fraction of RNA b molecules were bound in the complex; the bulk of RNA b sedimented at the same rate as the deproteinized RNA. Thus, after replication, maize mitochondrial plasmids are not associated with nucleoprotein capsids although their synthesis takes place through ribonucleoprotein replication complexes.
RNA b是在玉米线粒体中发现的自主复制单链和双链RNA质粒家族中含量最丰富的成员。通过对S细胞质玉米线粒体澄清裂解物进行速率区带离心和CsCl平衡密度梯度离心，研究了该分子与蛋白质的结合程度。鉴定出一种RNA b的可溶性复合物，它负责在线粒体裂解物中合成含量更丰富的(+)RNA b链。该复合物的浮力密度为1.49 g/cm³，表明其含有大量非核酸成分。然而，该复合物的沉降系数仅略大于脱蛋白的RNA b。RNA b以及更大的RNA质粒RNA a的合成对肝素具有抗性，这表明在裂解物中存在RNA模板与能够在体内延伸预先起始的RNA质粒链的RNA依赖性RNA聚合酶之间预先形成的复合物，这两种RNA均是如此。只有一小部分RNA b分子结合在复合物中；大部分RNA b的沉降速率与脱蛋白的RNA相同。因此，复制后，玉米线粒体质粒虽然通过核糖核蛋白复制复合物进行合成，但并不与核蛋白衣壳结合。
