In vivo mobility of a group I twintron in nuclear ribosomal DNA of the myxomycete Didymium iridis

Abstract

SummaryDiSSU1 is an optional group I twintron present in the nuclear extrachromosomal ribosomal DNA of the myxomycete Didymium iridis. DiSSU1 appears to be complex both in structure and function. At the RNA level it has a twin-ribozyme organization composed of two group I ribozymes with different functions, separated by an open reading frame. Here, we show that DiSSU1 is mobile when haploid intron-containing and intron-less amoebae are mated. The mobility process is fast, being completed in 5-10 nuclear cycles after mating in the developing zygote and plasmodia. Analyses of progeny from genetic crosses confirm intron mobility. DiSSU1 is the first example of a mobile group I twintron. The intron-encoded protein was expressed in Escherichia coli and found to be an endonuclease, I-Dir I, that cleaves an intron-less ribosomal DNA allele at the intron-insertion site, and is probably involved in intron homing. The endonuclease I-Dir I seems to be a rare example of a protein that is expressed from a ribozyme-processed RNA polymerase I transcript in vivo.

The Distribution of Group I Introns in Lichen Algae Suggests That Lichenization Facilitates Intron Lateral Transfer

“…Mobility at the DNA level may be facilitated by an endonuclease (ENase) that recognizes a long sequence (15-35 nt) within a homologous intronless copy of the coding region (Lambowitz and Belfort, 1993). Initiation of a double-strand break in the DNA of the intronless coding region is followed by a unidirectional conversion event that results in intron insertion (Belfort and Perlman, 1995;Johansen et al, 1997). This process, called intron ''homing,'' is highly efficient at introducing introns into the alleles of a gene but is not expected to lead to the frequent lateral transfer of group I introns into heterologous sites (intron transposition) because of the high endonuclease sequence specificity (Dujon, 1989; but see Bryk et al, 1993).…”

In vivo Expression of a Group I Intron HEG from the Antisense Strand of Didymium Ribosomal DNA

“…Cultivation of the D. iridis strain CR8-1, derived from the Costa Rica 8 isolate, and strain Hon1-7 (S7), derived from the Honduras 1 isolate, have previously been described. 12,14 The EMBL/GenBank accession numbers for SSU rDNA containing Dir.S956-1 group I intron (Intron 1, Pan 2-44), Dir.S956-2 group I intron (Intron 2, CR8-1) or lacking intron (Hon1-7) are AJ938153, AJ938154 and AJ938152, respectively. In order to isolate cellular RNA from D. iridis strains 10 7 amoeba were harvested by 5 min centrifugation at 400 g. The pellet was dissolved in 1 ml TRIzol Reagent (Gibco-BRL) and RNA extracted according to the manufacturers instructions.…”

I-PpoI, the Endonuclease Encoded by the Group I Intron PpLSU3, Is Expressed from an RNA Polymerase I Transcript

“…150 nuclear group I introns reported so far, only three have been shown or have been inferred to be mobile: DiSSU1 from the slime mold Didymium iridis (6,21,22), NaSSU1 from the protist Naegleria andersoni and other Naegleria species (8), and PpLSU3 from the slime mold Physarum polycephalum. Originally found in the large-subunit rDNA gene of the Carolina strain (29), PpLSU3 contains the open reading frame (ORF) for the homing endonuclease I-PpoI (for nomenclature of intron-encoded endonucleases, see reference 7) in its 5Ј half and the ribozyme part in its 3Ј half.…”