Gene transfer from organelles to the nucleus: How much, what happens, and why?
Gene transfer from organelles to the nucleus: How much, what happens, and why?
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DOI：
10.1104/pp.118.1.9
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发表时间：
1998-09-01
期刊：
影响因子：
7.4
通讯作者：
Herrmann, RG
中科院分区：
文献类型：
--
作者：
Martin, W;Herrmann, RG
叶绿体曾经是自由生活的蓝藻，线粒体曾经是自由生活的蛋白细菌，两者都保存了优生细菌基因组的残留物。但从功能的角度来看，这两个细胞器保留的优生菌生物化学比它们的DNA中反映的要多得多。细胞器编码的基因数量和它们所包含的真细菌蛋白质数量之间的差异通常可以用我们所熟知的内生共生基因转移来解释。在进化过程中，细胞器将它们的基因输出到细胞核，但在转运肽和蛋白质输入机制的帮助下重新输入产物，因此蛋白质保留在细胞器中，但大多数基因没有。随着时间的推移，这个过程会将遗传物质集中在核染色体中。由于细胞核控制下的基因调控过程比细胞器控制下的基因调控过程更加复杂和相互关联，而且细胞器自然倾向于受到核调控基因的控制(想象一下恰恰相反！)，细胞器调控过程很可能是第一批成功转移到细胞核的过程之一。因此，从基因的观点来看，这一过程导致在核的调控主导下形成一个分隔的但整合的真核遗传系统(Herrmann，1997)，而不是遗传上的半自治细胞器。然而，从转移基因的编码产物的角度来看，一幅令人惊讶的画面正在浮现，可以粗略地描述为“在返回细胞器的过程中发生了一件有趣的事情”。内生共生基因转移的先决条件是围绕叶绿体和线粒体的两层膜上的蛋白质输入机制，这使得这些细胞器能够吸收胞浆前体，裂解转运肽，并将处理后的多肽分别释放到基质和基质中。为了概述这种机制由什么蛋白质组成，它是如何工作的，以及它可能是如何进化的，我们推荐Schatz和Dobberstein(1996)最近的概述，特别是Heins等人(1998)对叶绿体的概述。
Chloroplasts were once free-living cyanobacteria, mitochondria were once free-living proteobacteria, and both have preserved remnants of eubacterial genomes. But from the functional standpoint, both organelles have retained much more of their eubacterial biochemistry than is reflected in their DNA. The discrepancy between the number of genes that organelles encode and the number of eubacterial proteins that they contain is generally explained by something that we have come to know as “endosymbiotic gene transfer.” During evolution, organelles export their genes to the nucleus, but reimport the products with the help of transit peptides and protein-import machinery, so that proteins are retained in organelles, but most of the genes are not. This process, over time, concentrates genetic material in nuclear chromosomes. Because gene-regulatory processes under the control of the nucleus are more complex and interrelated than those under the control of organelles, and because organelles naturally tend to come under the control of nuclear regulatory genes (imagine the opposite!), organelle regulatory processes are likely to have been among the first to be transferred successfully to the nucleus. From the standpoint of genes, this process therefore results in a compartmented, but integrated, eukaryotic genetic system under the regulatory dominance of the nucleus (Herrmann, 1997), rather than genetically semiautonomous organelles. However, from the standpoint of the encoded products of transferred genes, a surprising picture is emerging that could be loosely described as “a funny thing happened on the way back to the organelle.” The prerequisite for endosymbiotic gene transfer is protein-import machinery in the two membranes that surround chloroplasts and mitochondria, which allows these organelles to take up cytosolic precursors, cleave the transit peptides, and release the processed polypeptides into the stroma and matrix, respectively. For an overview of what proteins that machinery consists of, how it works, and how it might have evolved, we recommend the recent overviews by Schatz and Dobberstein (1996) for a general summary, and Heins et al.(1998) for chloroplasts in particular.