title: Positions of multiple insertions in SSU rDNA of lichen-forming fungi.
abstract: Lichen-forming fungi, in symbiotic associations with algae, frequently have nuclear small subunit ribosomal DNA (SSU rDNA) longer than the 1,800 nucleotides typical for eukaryotes. The lichen-forming ascomycetous fungus Lecanora dispersa contains insertions at eight distinct positions of its SSU rDNA; the lichen-forming fungi Calicium tricolor and Porpidia crustulata each contain one insertion. Insertions are not limited to fungi that form lichens; the lichen ally Mycocalicium albonigrum also contains two insertions. Of the 11 insertion positions now reported for lichen-forming fungi and this ally, 6 positions are known only from lichen-forming fungi. Including the 4 newly reported in this study, insertions are now known from at least 17 positions among all reported SSU rDNA sequences. Insertions, most of which are Group I introns, are reported in fungal and protistan lineages and occur at corresponding positions in genomes as phylogenetically distant as the nuclei of fungi, green algae, and red algae. Many of these positions are exposed in the mature rRNA tertiary structure and may be subject to independent insertion of introns. Insertion of introns, accompanied by their sporadic loss, accounts for the scattered distribution of insertions observed within the SSU rDNA of these diverse organisms.
authors.affiliation: Department of Botany, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560.
authors.forename: A
authors.initials: A
authors.lastname: Gargas
authors.forename: P T
authors.initials: PT
authors.lastname: DePriest
authors.forename: J W
authors.initials: JW
authors.lastname: Taylor
mesh_terms: D001483:Base Sequence; D005075:Biological Evolution; D004251:DNA Transposable Elements / Q000235:genetics*; D004271:DNA, Fungal / Q000235:genetics*; D005658:Fungi / Q000235:genetics*; D008011:Lichens / Q000235:genetics*; D008969:Molecular Sequence Data; D010802:Phylogeny; D012335:RNA, Ribosomal / Q000235:genetics*; D017422:Sequence Analysis, DNA
chemical_list: D004251:DNA Transposable Elements; D004271:DNA, Fungal; D012335:RNA, Ribosomal