Accumulation of Stable Full-Length Circular Group I Intron RNAs during Heat-Shock.
Group I introns in nuclear ribosomal RNA of eukaryotic microorganisms are processed by splicing or circularization. The latter results in formation of full-length circular introns without ligation of the exons and has been proposed to be active in intron mobility. We applied qRT-PCR to estimate the copy number of circular intron RNA from the myxomyceteDidymium iridis. In exponentially growing amoebae, the circular introns are nuclear and found in 70 copies per cell. During heat-shock, the circular form is up-regulated to more than 500 copies per cell. The intron harbours two ribozymes that have the potential to linearize the circle. To understand the structural features that maintain circle integrity, we performed chemical and enzymatic probing of the splicing ribozyme combined with molecular modeling to arrive at models of the inactive circular form and its active linear counterpart. We show that the two forms have the same overall structure but differ in key parts, including the catalytic core element P7 and the junctions at which reactions take place. These differences explain the relative stability of the circular species, demonstrate how it is prone to react with a target molecule for circle integration and thus supports the notion that the circular form is a biologically significant molecule possibly with a role in intron mobility.
Department of Cellular and Molecular Medicine, The Panum Institute, University of Copenhagen, DK-2200 Copenhagen N, Denmark. kasper.andersen@bric.ku.dk.
Kasper L
KL
Andersen
Department of Cellular and Molecular Medicine, The Panum Institute, University of Copenhagen, DK-2200 Copenhagen N, Denmark. beckert@genzentrum.lmu.de.|Molecular Genetics Genomics Microbiology, Université de Strasbourg, CNRS, UMR 7156, Strasbourg 67081, France. beckert@genzentrum.lmu.de.
Bertrand
B
Beckert
Molecular Genetics Genomics Microbiology, Université de Strasbourg, CNRS, UMR 7156, Strasbourg 67081, France. b.masquida@unistra.fr.
Benoit
B
Masquida
Department of Medical Biology, UiT, The Arctic University of Norway, Tromsø N-9037, Norway. steinar.johansen@uit.no.
Steinar D
SD
Johansen
Department of Cellular and Molecular Medicine, The Panum Institute, University of Copenhagen, DK-2200 Copenhagen N, Denmark. hamra@sund.ku.dk.
Henrik
H
Nielsen
D018869:Heat-Shock Response / Q000502:physiology*; D007438:Introns; D009235:Myxomycetes / Q000235:genetics / Q000378:metabolism*; D016337:RNA, Catalytic / Q000096:biosynthesis* / Q000235:genetics
D016337:RNA, Catalytic
Didymium iridis; RNA catalysis; circular RNA; group I intron; horizontal gene transfer; molecular modeling