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A Deviant Genetic Code in the Reduced Mitochondrial Genome of the Picoplanktonic Green Alga Pycnococcus provasolii

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Abstract

Reduction in size of flagellated chlorophytes occurred multiple times during evolution, providing the opportunity to study the consequences of cell reduction on genome architecture. Recent investigations on the chloroplast genomes of the tiny prasinophyceans Ostreococcus tauri (Mamiellales), Micromonas sp. RCC299 (Mamiellales), and Pycnococcus provasolii (Pseudocourfieldiales) highlighted their extreme compaction and reduced gene repertoires. Genome compaction is also exemplified by the Ostreococcus and Micromonas mitochondrial DNAs (mtDNAs) although they have retained almost all of the about 65 genes presumably present in the mitochondria of ancestral prasinophyceans. In this study, the mitochondrial genome of Pycnococcus was sequenced and compared to those of previously examined chlorophytes. Our results document the first case where cellular reduction of a free-living alga was accompanied by marked reduction in gene content of both the mitochondrial and chloroplast genomes. At 24,321 bp, the intronless Pycnococcus mitochondrial genome falls within the lower size range displayed by green algal mtDNAs. The 36 conserved genes, specifying two rRNAs with conventional structures, 16 tRNAs and 18 proteins, are all encoded on the same DNA strand and represent 88% of the genome. Besides a pronounced codon bias, the protein-coding genes feature a variant genetic code characterized by the use of TGA (normally a stop codon) to code for tryptophan, and the unprecedented use of TTA and TTG (normally leucine codons) as stop codons. We conclude that substantial reduction of the mitochondrial genome occurred in at least three independent chlorophyte lineages and that this process entailed a number of convergent changes in these lineages.

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Acknowledgments

This work was supported by a grant from the Natural Sciences and Engineering Research Council of Canada (to MT and CL). The Pycnococcus mitochondrial genome sequence reported in this study has been deposited in GenBank under the accession number GQ497137.

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  1. Département de biochimie et de microbiologie, Université Laval, 1030 avenue de la Médecine, Quebec, QC, G1V 0A6, Canada

    Monique Turmel, Christian Otis & Claude Lemieux

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  1. Monique Turmel
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  2. Christian Otis
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  3. Claude Lemieux
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Correspondence to Monique Turmel.

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Turmel, M., Otis, C. & Lemieux, C. A Deviant Genetic Code in the Reduced Mitochondrial Genome of the Picoplanktonic Green Alga Pycnococcus provasolii . J Mol Evol 70, 203–214 (2010). https://doi.org/10.1007/s00239-010-9322-6

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  • DOI: https://doi.org/10.1007/s00239-010-9322-6

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