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BMC Genomics. 2009; 10: 528.
Published online Nov 16, 2009. doi:  10.1186/1471-2164-10-528
PMCID: PMC2784804
U3 snoRNA genes are multi-copy and frequently linked to U5 snRNA genes in Euglena gracilis§
J Michael Charette1,2 and Michael W Graycorresponding author1,3
1Centre for Comparative Genomics and Evolutionary Bioinformatics, Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, Nova Scotia, Canada
2Current address: Department of Molecular Biophysics & Biochemistry, Yale University School of Medicine, Sterling Hall of Medicine, 333 Cedar St, Room SHM C-114, PO Box 208024, New Haven, CT 06520-8024, USA
3Correspondence address: Room 8-F2, Sir Charles Tupper Medical Building, Dalhousie University, 5850 College Street, Halifax, B3H 1X5, Nova Scotia, Canada
corresponding authorCorresponding author.
J Michael Charette: Michael.Charette/at/Yale.edu; Michael W Gray: m.w.gray/at/dal.ca
Received August 28, 2009; Accepted November 16, 2009.
Abstract
Background
U3 snoRNA is a box C/D small nucleolar RNA (snoRNA) involved in the processing events that liberate 18S rRNA from the ribosomal RNA precursor (pre-rRNA). Although U3 snoRNA is present in all eukaryotic organisms, most investigations of it have focused on fungi (particularly yeasts), animals and plants. Relatively little is known about U3 snoRNA and its gene(s) in the phylogenetically broad assemblage of protists (mostly unicellular eukaryotes). In the euglenozoon Euglena gracilis, a distant relative of the kinetoplastid protozoa, Southern analysis had previously revealed at least 13 bands hybridizing with U3 snoRNA, suggesting the existence of multiple copies of U3 snoRNA genes.
Results
Through screening of a λ genomic library and PCR amplification, we recovered 14 U3 snoRNA gene variants, defined by sequence heterogeneities that are mostly located in the U3 3'-stem-loop domain. We identified three different genomic arrangements of Euglena U3 snoRNA genes: i) stand-alone, ii) linked to tRNAArg genes, and iii) linked to a U5 snRNA gene. In arrangement ii), the U3 snoRNA gene is positioned upstream of two identical tRNAArg genes that are convergently transcribed relative to the U3 gene. This scenario is reminiscent of a U3 snoRNA-tRNA gene linkage previously described in trypanosomatids. We document here twelve different U3 snoRNA-U5 snRNA gene arrangements in Euglena; in each case, the U3 gene is linked to a downstream and convergently oriented U5 gene, with the intergenic region differing in length and sequence among the variants.
Conclusion
The multiple U3 snoRNA-U5 snRNA gene linkages, which cluster into distinct families based on sequence similarities within the intergenic spacer, presumably arose by genome, chromosome, and/or locus duplications. We discuss possible reasons for the existence of the unusually large number of U3 snoRNA genes in the Euglena genome. Variability in the signal intensities of the multiple Southern hybridization bands raises the possibility that Euglena contains a naturally aneuploid chromosome complement.
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