Dissecting translation in the chloroplast

• Main Author: Manuell, Andrea Lynne
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2007
• Subjects: Biophysics; Molecular biology
• ISBN: 0549109935; 9780549109938

Chloroplasts are both the basis for oxygenic life on this earth, and a modern target for biotechnologies (e.g. BioFuels, recombinant protein production). The primary metabolic function of the chloroplast is photosynthesis, which employs a number of membrane-bound protein complexes composed of both nuclear- and chloroplast-encoded subunits. Other important complexes in the chloroplast, like the ribosome, are also composed of gene products from both genomes, and render a communication between the genomes key for formation and maintenance of these complexes. The positive translational regulation of chloroplast genes through a requirement for the presence or function of nuclear gene products allows for coordinate expression with the nuclear genome. Many nuclear-encoded chloroplast-targeted gene products are required for the translation of chloroplast mRNAs. In many cases, these proteins bind the 5' untranslated region of a plastid mRNA, often in complexes. These assembled translation initiation mRNP complexes communicate that an mRNA is translatable. The mechanisms of this communication, and whether it is accomplished by the proteins or the mRNA, are poorly understood. The chloroplast ribosome is modified from a bacterial ribosome through the addition of chloroplast-unique protein components. We now visualize these chloroplast-unique features on the three-dimensional structure of the chloroplast ribosome from C. reinhardtii solved using cryo-electron microscopy and single-particle reconstruction. These structures are found near the areas of mRNA interaction with the ribosome, and do not disturb, but add to, the basic form of the 70S ribosome. With the wealth of knowledge on the structures and mechanisms of translation in bacteria, we are able to hypothesize the involvement of these chloroplast-unique structures specifically in translation initiation. Our model for chloroplast translation is that the protein and mRNA components of the translation initiation mRNP complexes must interact with the chloroplast-unique components of the ribosome. These interactions position mRNAs appropriately for translation initiation, which apparently can not be done by just the mRNA and the chloroplast ribosome alone. Translation then proceeds as in bacteria, excepting the programmed pausing of translation of important integral membrane proteins. These specializations are in sharp contrast to translation in the cytoplasm of C. reinhardtii, which proceeds on a ribosome that is very highly conserved with 80S ribosomes from other studied organisms like yeast and mammals. This contrast between conservation and specialization in the cytoplasm and organelle, combined with a wealth of knowledge on ribosome form and function from other systems, allows us to fit the chloroplast ribosome into the larger picture of translation. A thorough understanding of the basic mechanisms of chloroplast ribosome function and mRNA selection will complement the current status of ribosome knowledge. It may also allow for the more appropriate design of biotechnological endeavors that aim to exploit the chloroplast for expression of industrial and pharmaceutically useful proteins.