Henrik Nielsen

In vivo expression of the nucleolar group I intron‐encoded I‐DirI homing endonuclease involves the removal of a spliceosomal intron

The Didymium iridis DiSSU1 intron is located in the nuclear SSU rDNA and has an unusual twin‐ribozyme organization. One of the ribozymes (DiGIR2) catalyses intron excision and exon ligation. The other ribozyme (DiGIR1), which along with the endonuclease‐encoding I‐DirI open reading frame (ORF) is inserted in DiGIR2, carries out hydrolysis at internal processing sites (IPS1 and IPS2) located at its 3′ end. Examination of the in vivo expression of DiSSU1 shows that after excision, DiSSU1 is matured further into the I‐DirI mRNA by internal DiGIR1‐catalysed cleavage upstream of the ORF 5′ end, as well as truncation and polyadenylation downstream of the ORF 3′ end. A spliceosomal intron, the first to be reported within a group I intron and the rDNA, is removed before the I‐DirI mRNA associates with the polysomes. Taken together, our results imply that DiSSU1 uses a unique combination of intron‐supplied ribozyme activity and adaptation to the general RNA polymerase II pathway of mRNA expression to allow a protein to be produced from the RNA polymerase I‐transcribed rDNA.

Complex group-I introns in nuclear SSU rDNA of red and green algae: evidence of homing-endonuclease pseudogenes in the Bangiophyceae

Abstract The green alga Scenedesmus pupukensis and the red alga Porphyra spiralis contain large group-IC1 introns in their nuclear small subunit ribosomal RNA genes due to the presence of open reading frames at the 5′ end of the introns. The putative 555 amino-acid Scenedesmus-encoded protein harbors a sequence motif resembling the bacterial S9 ribosomal proteins. The Porphyra intron self-splices in vitro, and generates both ligated exons and a full-length intron RNA circle. The Porphyra intron has an unusual structural organization by encoding a potential 149 amino-acid homing-endonuclease-like protein on the complementary strand. A comparison between related group-I introns in the Bangiophyceae revealed homing-endonuclease-like pseudogenes due to frame-shifts and deletions in Porphyra and Bangia. The Scenedesmus and Porphyra introns provide new insights into the evolution and possible novel functions of nuclear group-I intron proteins.

DNA from Arctic Human Burials

The climate of Arctic and Alpine areas should facilitate the recovery of DNA from human and animal remains. A priori, one would expect postmortem processes as well as subsequent microbial decomposition to be affected favorably by the low temperatures and the low humidity characteristic of these areas. In this chapter we will discuss this assumption and describe some finds of frozen remains.

8-METHOXYPSORALEN DNA INTERSTRAND CROSS-LINKING OF THE RIBOSOMAL RNA GENES IN Tetrahymena thermophila. DISTRIBUTION, REPAIR AND EFFECT ON rRNA SYNTHESIS

Abstract— The distribution and repair of 8‐methoxypsoralen‐DNA interstrand cross‐links in the ribosomal RNA gencs (rDNA) in Tetrahymena thermophila have been studied in vivo by Southern blot analysis. It is found that the cross‐links at a density of ≤ 1/2 × 104 base pairs (bp) are distributed equally between three domains (terminal spacer, transcribed region and central spacer) as defined by restriction enzyme analysis (Bam HI and ClaI). It is furthermore shown that a dosage resulting in approximately one cross‐link per rDNA molecule (21 kbp, two genes) is suficient to block KNA synthesis. Finally, it is shown that the cross‐links in the rDNA molecules are repaired at equal rate in all three domains within 24 h and that RNA synthesis is partly restored during this repair period. The majority of the cells also go through one to two cell divisions in this period but do not survive.

Chapter 12:The GIR1 Branching Ribozyme

The GIR1 branching ribozyme is a ca. 179 nt ribozyme with structural resemblance to group I ribozymes.1 It is found within a complex type of group I introns, termed the twin-ribozyme introns.2 Rather than splicing, it catalyses a branching reaction in which the 2′OH of an internal residue is involve...