No abstract

Chloroplast subfractions were tested with a UV cross-linking assay for proteins that bind to the 5′ untranslated region of the chloroplast psbC mRNA of the green alga Chlamydomonas reinhardtii. These analyses revealed that RNA-binding proteins of 30–32, 46, 47, 60, and 80 kD are associated with chloroplast membranes. The buoyant density and the acyl lipid composition of these membranes are compatible with their origin being the inner chloroplast envelope membrane. However, unlike previously characterized inner envelope membranes, these membranes are associated with thylakoids. One of the membrane-associated RNA-binding proteins appears to be RB47, which has been reported to be a specific activator of psbA mRNA translation. These results suggest that translation of chloroplast mRNAs encoding thylakoid proteins occurs at either a subfraction of the chloroplast inner envelope membrane or a previously uncharacterized intra-chloroplast compartment, which is physically associated with thylakoids.

In the green alga Chiamydomonas reinhardtii, the nuclear mutations F34 and F64 have been previously shown to abolish the synthesis of the photosystem II core polypeptide subunit P6, which is encoded by the chloroplast psbC gene. In this report the functions encoded by F34 and F64 are synthesized within the chloroplast stroma on 70S ribosomes. Expression of the correct stochiometric ratios of these polypeptides is thought to be governed by genetic circuitry between the genomes of these two compartments (reviewed in references 16 and 30). The chloroplast transcription and translation systems are similar to those of eubacteria, reflecting the procaryotic ancestry of chloroplasts. However, there are differences between the two systems. With only a few exceptions, the translation initiation sites in Escherichia coli contain Shine-Dalgarno sequences within the 15 nucleotides 5' to the initiation codon (38). Of the initiation codons of chloroplast genes, however, only 40% have a potential Shine-Dalgarno sequence in this region (2). Also, five chloroplast ribosomal proteins have no known counterparts in eubacteria (40). Chloroplast gene expression requires many nuclear genes, most of which encode unknown functions (30). Thus, the expression and regulation of chloroplast genes probably differ in many respects from the expression and regulation of procaryotic genes or the nuclear genes of eucaryotes.The unicellular green alga Chlamydomonas reinhardtii is an excellent experimental system for the study of chloroplast genetics, primarily because of the viability of photosynthesis-* Corresponding author. Mailing address:

A subunit of the chloroplast pyruvate dehydrogenase complex, which serves as a metabolic enzyme, also has a dual function as an RNA-binding protein and influences mRNA translation.

Cells have complex membranous organelles for the compartmentalization and the regulation of most intracellular processes. Organelle biogenesis and maintenance requires newly synthesized proteins, each of which needs to go from the ribosome translating its mRNA to the correct membrane for insertion or transclocation to an a organellar subcompartment. Decades of research have revealed how proteins are targeted to the correct organelle and translocated across one or more organelle membranes ro the compartment where they function. The paradigm examples involve interactions between a peptide sequence in the protein, localization factors, and various membrane embedded translocation machineries. Membrane translocation is either cotranslational or posttranslational depending on the protein and target organelle. Meanwhile research in embryos, neurons and yeast revealed an alternative targeting mechanism in which the mRNA is localized and only then translated to synthesize the protein in the correct location. In these cases, the targeting information is coded by the cis-acting sequences in the mRNA ("Zipcodes") that interact with localization factors and, in many cases, are transported by the molecular motors on the cytoskeletal filaments. Recently, evidence has been found for this "mRNA based" mechanism in organelle protein targeting to endoplasmic reticulum, mitochondria, and the photosynthetic membranes within chloroplasts. Here we review known and potential roles of mRNA localization in protein targeting to and within organelles. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.