title: Type I-like intervening sequences are found in the rDNA of the nematode Ascaris lumbricoides.
abstract: The intervening sequences in the large ribosomal RNA gene of Ascaris lumbricoides var. suum show many similarities to the type I insertions, previously found only in some insect species. They include structural features, but also a presumed transcriptional inactivity in vivo: No transcript of the rDNA intervening sequence in A. lumbricoides could be detected in Northern and dot blot hybridizations. However, the primary structure of the Pol I promoter region is well conserved in interrupted and uninterrupted genes. Moreover, genes with an intervening sequence are correctly initiated in a whole-cell in vitro extract from Ascaris oogonia. Hence, the presence of the intervening sequence alone does not seem to account for a transcriptional inhibition in rRNA genes. As with the type I insertions of insect rDNA, some copies of the A. lumbricoides intervening sequence are also present in locations outside the rDNA cluster. About 50% of the extraribosomal copies are found in a repetitive sequence of the genome, and additional copies are inserted in unique sequences. These striking analogies to type I insertions are discussed, and lead to the conclusion that the two phenomena are undoubtedly related. This is the first report proving the presence of a type I-like insertion element outside of the class Insecta.
authors.affiliation: Institute of Zoology, University of Fribourg, Switzerland.
authors.forename: H
authors.initials: H
authors.lastname: Neuhaus
authors.forename: F
authors.initials: F
authors.lastname: Müller
authors.forename: A
authors.initials: A
authors.lastname: Etter
authors.forename: H
authors.initials: H
authors.lastname: Tobler
mesh_terms: D000818:Animals; D001200:Ascaris / Q000235:genetics*; D001483:Base Sequence; D004251:DNA Transposable Elements; D004275:DNA, Ribosomal / Q000235:genetics*; D004331:Drosophila melanogaster / Q000235:genetics; D005796:Genes; D008969:Molecular Sequence Data; D011401:Promoter Regions, Genetic; D012335:RNA, Ribosomal / Q000096:biosynthesis; D012091:Repetitive Sequences, Nucleic Acid; D014158:Transcription, Genetic
chemical_list: D004251:DNA Transposable Elements; D004275:DNA, Ribosomal; D012335:RNA, Ribosomal