The miniaturized nuclear genome of eukaryotic endosymbiont contains genes that overlap, genes that are cotranscribed, and the smallest known spliceosomal introns.
Chlorarachniophyte algae contain a complex, multi-membraned chloroplast derived from the endosymbiosis of a eukaryotic alga. The vestigial nucleus of the endosymbiont, called the nucleomorph, contains only three small linear chromosomes with a haploid genome size of 380 kb and is the smallest known eukaryotic genome. Nucleotide sequence data from a subtelomeric fragment of chromosome III were analyzed as a preliminary investigation of the coding capacity of this vestigial genome. Several housekeeping genes including U6 small nuclear RNA (snRNA), ribosomal proteins S4 and S13, a core protein of the spliceosome [small nuclear ribonucleoprotein (snRNP) E], and a cip-like protease (clpP) were identified. Expression of these genes was confirmed by combinations of Northern blot analysis, in situ hybridization, immunocytochemistry, and cDNA analysis. The protein-encoding genes are typically eukaryotic in overall structure and their messenger RNAs are polyadenylylated. A novel feature is the abundance of 18-, 19-, or 20-nucleotide introns; the smallest spliceosomal introns known. Two of the genes, U6 and S13, overlap while another two genes, snRNP E and clpP, are cotranscribed in a single mRNA. The overall gene organization is extraordinarily compact, making the nucleomorph a unique model for eukaryotic genomics.
Gilson
P R
PR
Plant Cell Biology Research Centre, School of Botany, University of Melbourne, Australia.
McFadden
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GI
D000656:Amoeba / Q000145:classification / Q000235:genetics*; D000818:Animals; D001483:Base Sequence; D000460:Chlorophyta / Q000145:classification / Q000235:genetics*; D016384:Consensus Sequence; D017124:Conserved Sequence; D019143:Evolution, Molecular; D017403:In Situ Hybridization; D007438:Introns; D008854:Microscopy, Electron; D008969:Molecular Sequence Data; D016366:Open Reading Frames; D018087:Plastids; D016133:Polymerase Chain Reaction; D014176:Protein Biosynthesis; D012342:RNA, Small Nuclear / Q000032:analysis / Q000096:biosynthesis*; D015183:Restriction Mapping; D012689:Sequence Homology, Nucleic Acid; D017381:Spliceosomes / Q000648:ultrastructure
D012342:RNA, Small Nuclear