title: Sequence and structure of the extrachromosomal palindrome encoding the ribosomal RNA genes in Dictyostelium.
abstract: Ribosomal RNAs (rRNAs) are encoded by multicopy families of identical genes. In Dictyostelium and other protists, the rDNA is carried on extrachromosomal palindromic elements that comprise up to 20% of the nuclear DNA. We present the sequence of the 88 kb Dictyostelium rDNA element, noting that the rRNA genes are likely to be the only transcribed regions. By interrogating a library of ordered YAC clones, we provide evidence for a chromosomal copy of the rDNA on chromosome 4. This locus may provide master copies for the stable transmission of the extrachromosomal elements. The extrachromosomal elements were also found to form chromosome-sized clusters of DNA within nuclei of nocodazole-treated cells arrested in mitosis. These clusters resemble true chromosomes and may allow the efficient segregation of the rDNA during mitosis. These rDNA clusters may also explain the cytological observations of a seventh chromosome in this organism.
authors.affiliation: Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, TX 77030, USA.
authors.forename: Richard
authors.initials: R
authors.lastname: Sucgang
authors.forename: Guokai
authors.initials: G
authors.lastname: Chen
authors.forename: Wen
authors.initials: W
authors.lastname: Liu
authors.forename: Ryan
authors.initials: R
authors.lastname: Lindsay
authors.forename: Jing
authors.initials: J
authors.lastname: Lu
authors.forename: Donna
authors.initials: D
authors.lastname: Muzny
authors.forename: Gad
authors.initials: G
authors.lastname: Shaulsky
authors.forename: William
authors.initials: W
authors.lastname: Loomis
authors.forename: Richard
authors.initials: R
authors.lastname: Gibbs
authors.forename: Adam
authors.initials: A
authors.lastname: Kuspa
mesh_terms: D000818:Animals; D002874:Chromosome Mapping; D016054:DNA, Protozoan / Q000737:chemistry / Q000235:genetics; D004023:Dictyostelium / Q000235:genetics*; D017404:In Situ Hybridization, Fluorescence; D008969:Molecular Sequence Data; D010957:Plasmids / Q000235:genetics; D012335:RNA, Ribosomal / Q000235:genetics*; D017422:Sequence Analysis, DNA
chemical_list: D016054:DNA, Protozoan; D012335:RNA, Ribosomal