title: Cassandra retrotransposons carry independently transcribed 5S RNA.
abstract: We report a group of TRIMs (terminal-repeat retrotransposons in miniature), which are small nonautonomous retrotransposons. These elements, named Cassandra, universally carry conserved 5S RNA sequences and associated RNA polymerase (pol) III promoters and terminators in their long terminal repeats (LTRs). They were found in all vascular plants investigated. Uniquely for LTR retrotransposons, Cassandra produces noncapped, polyadenylated transcripts from the 5S pol III promoter. Capped, read-through transcripts containing Cassandra sequences can also be detected in RNA and in EST databases. The predicted Cassandra RNA 5S secondary structures resemble those for cellular 5S rRNA, with high information content specifically in the pol III promoter region. Genic integration sites are common for Cassandra, an unusual feature for abundant retrotransposons. The 5S in each LTR produces a tandem 5S arrangement with an inter-5S spacing resembling that of cellular 5S. The distribution of 5S genes is very variable in flowering plants and may be partially explained by Cassandra activity. Cassandra thus appears both to have adapted a ubiquitous cellular gene for ribosomal RNA for use as a promoter and to parasitize an as-yet-unidentified group of retrotransposons for the proteins needed in its lifecycle.
authors.affiliation: MTT/BI Plant Genomics Laboratory, Institute of Biotechnology, Viikki Biocenter, University of Helsinki, FIN-00014, Helsinki, Finland.
authors.forename: Ruslan
authors.initials: R
authors.lastname: Kalendar
authors.forename: Jaakko
authors.initials: J
authors.lastname: Tanskanen
authors.forename: Wei
authors.initials: W
authors.lastname: Chang
authors.forename: Kristiina
authors.initials: K
authors.lastname: Antonius
authors.forename: Hanan
authors.initials: H
authors.lastname: Sela
authors.forename: Ofer
authors.initials: O
authors.lastname: Peleg
authors.forename: Alan H
authors.initials: AH
authors.lastname: Schulman
mesh_terms: D001483:Base Sequence; D017343:Genes, Plant; D018745:Genome, Plant; D008969:Molecular Sequence Data; D012320:RNA Polymerase III; D012341:RNA, Ribosomal, 5S; D018626:Retroelements; D020079:Terminal Repeat Sequences; D014158:Transcription, Genetic
chemical_list: D012341:RNA, Ribosomal, 5S; D018626:Retroelements; D012320:RNA Polymerase III