Westhoff P, Herrmann R G
Botanisches Institut, Universität München, Federal Republic of Germany.
Eur J Biochem. 1988 Feb 1;171(3):551-64. doi: 10.1111/j.1432-1033.1988.tb13824.x.
The psbB operon of the spinach plastid chromosome encodes the genes for the 51-kDa chlorophyll a apoprotein (psbB), the 10-kDa phosphoprotein (psbH), both associated with photosystem II, as well as cytochrome b6 (petB) and subunit IV (petD) of the cytochrome b/f complex in the order given. These genes are not expressed coordinately. The RNA pattern of this DNA region is complex and resolves into eighteen major RNA species. Using northern and S1 protection analysis we demonstrate (a) that all RNA species derive from one DNA strand and hybridize in an overlapping fashion; and (b) that they arise by processing rather than by multiple transcription initiation/termination. (c) The operon is bordered by a single prokaryote-like promotor in front of psbB, and by a putative factor-independent terminator with characteristic sequence elements following petD. The terminator appears to function bidirectionally. (d) At least four distinct modification activities operate on the putative primary transcript of 5650 nucleotides and on the processing intermediates, including a novel endonucleolytic activity cleaving within a characteristic hexanucleotide motif, 3'-exonucleolytic activity at discrete RNA ends, 5' shortage of mRNA (psbB), and excision of class II intervening sequences (petB and petD). (e) Kinetically, maturation of the primary transcript is largely a stochastic process. (f) Processing results ultimately in the formation of monocistronic mRNAs for each of the two photosystem II polypeptides and a bicistronic mRNA encoding both subunits of the cytochrome b/f complex. We postulate that these RNA species represent the translationally active components in the non-coordinate dark/light expression of these genes. (g) Light is without any noticeable effect on posttranscriptional modification. Under our conditions it appears to operate at a translational rather than a transcriptional or posttranscriptional level indicating that the biogenesis of thylakoid membranes is regulated at various levels.
菠菜质体染色体的psbB操纵子按给定顺序编码与光系统II相关的51 kDa叶绿素a脱辅基蛋白（psbB）、10 kDa磷蛋白（psbH）的基因，以及细胞色素b/f复合体的细胞色素b6（petB）和亚基IV（petD）。这些基因并非协同表达。该DNA区域的RNA模式复杂，可解析为18种主要RNA种类。利用Northern印迹和S1核酸酶保护分析，我们证明：（a）所有RNA种类均源自一条DNA链，并以重叠方式杂交；（b）它们是通过加工产生的，而非多个转录起始/终止产生；（c）该操纵子在psbB之前由一个单一的原核生物样启动子界定，在petD之后由一个具有特征性序列元件的推定的不依赖因子的终止子界定。该终止子似乎双向发挥作用；（d）至少四种不同的修饰活性作用于推定的5650个核苷酸的初级转录本及其加工中间体，包括一种在特征性六核苷酸基序内切割的新型内切核酸酶活性、离散RNA末端的3'外切核酸酶活性、mRNA（psbB）的5'短缺以及II类间隔序列（petB和petD）的切除；（e）从动力学角度看，初级转录本的成熟在很大程度上是一个随机过程；（f）加工最终导致为两种光系统II多肽中的每一种形成单顺反子mRNA，以及一种编码细胞色素b/f复合体两个亚基的双顺反子mRNA。我们推测这些RNA种类代表了这些基因在非协同暗/光表达中具有翻译活性的成分；（g）光对转录后修饰没有任何明显影响。在我们的条件下，它似乎在翻译水平而非转录或转录后水平发挥作用，这表明类囊体膜的生物合成在多个水平上受到调控。
