Borrowed Chloroplasts: Secondary Endosymbiosis and the Chromalveolates
Lane, Christopher , van den Heuvel, Krystal , Curtis, Bruce , Fong, Anna , Kozera, Catherine , Parsons, Byron , Bowman, Sharen , Archibald, John .
Complete nucleomorph genome sequence of the cryptophyte alga Hemiselmis rufescens: insight into the causes and consequences of genome reduction in eukaryotes.
The cryptophytes are a poorly studied group of unicellular aquatic algae that acquired photosynthesis through secondary endosymbiosis, a process that occurs when a phagotrophic eukaryote engulfs a photosynthetic eukaryote and retains its plastid. Cryptophytes are unusual in that they retain the red-algal derived nucleus of their eukaryotic endosymbiont in a miniaturized form: the nucleomorph. Here we present the complete nucleomorph genome sequence of the cryptophyte Hemiselmis rufescens. The genome is 571,597 bp in size and is comprised of three similarly sized chromosomes (~207, 184 and 179 Kbp). With the goal of better understanding the process of genome reduction and compaction in eukaryotes, we have compared the structure and coding capacity of the H. rufescens sequence to the only other cryptophyte nucleomorph genome sequenced thus far, the ~551 Kbp genome of Guillardia theta. The overall structure of the two genomes is quite similar, although sub-telomeric ribosomal DNA operons are notably absent from both ends of chromosome II and one end of chromosome III in H. rufescens. Spliceosomal introns (and genes encoding spliceosomal proteins) have been completely lost in H. rufescens. Comparison of the sizes of nucleomorph-encoded proteins in H. rufescens and G. theta to one another and to homologs encoded in unreduced genomes indicate that (1) most G. theta proteins are smaller than their counterparts in H. rufescens and (2) nucleomorph proteins are smaller than their homologs in red and green algal genomes. The significance of these observations will be discussed.
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1 - Dalhousie University, Department of Biochemistry and Molecular Biology, 5850 College St, Halifax, NS, B3H1X5, Canada
2 - The Atlantic Genome Centre, Halifax, NS, Canada
Presentation Type: Symposium or Colloquium Presentation
Location: Waldorf Room/Hilton
Date: Wednesday, July 11th, 2007
Time: 9:00 AM