Unintelligent design: the regulation of plastid translation – University of Copenhagen

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Unintelligent design: the regulation of plastid translation

Plastids are one of the three compartments in plant cells that are able to synthesize their own proteins. Even though only around 80 genes are translated, they can make up to 50% of the protein content of plant cells. Translation is the major level of regulation of plastid genes. This regulation takes place during the development of the photosynthetic organs and also in response to changing light quantities and other abiotic stimuli. The translational machinery in plastids is a genetic mosaic composed of genes derived both from the cyanobacterial ancestor of the plastids as well as of genes of eukaryotic origin. The ribosomes and many of their assembly factors are of the bacterial type. Also the codon-anti-codon interaction is similar to bacteria, although plastids use a reduced set of tRNAs, which is not sufficient according to Crick’s wobble rules for decoding the genetic code.  Decoding depends partially on superwobbling instead. Many genes are part of operons and also the cis-elements required for translation initiation are similar to bacteria. But as in many parasitic and symbiotic bacterial species living in eukaryotic cells also a large part of the plastid genes does not possess classical Shine-Dalgarno elements. In contrast the regulation of translation depends on eukaryotic factors. These factors are mainly from protein families, which massively expanded in higher plants. How these factors regulate translation is part of ongoing research.  The translational apparatus in plastids is an interesting example of “unintelligent design” requiring more genes for its own synthesis and regulation than it is translating.