The use of PET-CT for preoperative staging of NSCLC reduced both the total number of thoracotomies and the number of futile thoracotomies but did not affect overall mortality. (ClinicalTrials.gov number, NCT00867412.)

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Group I introns are genetic elements interrupting functional genes. They are removed from precursors at the RNA level and most catalyze their own splicing. The catalytic part of these constitutes one of the major classes of catalytic RNAs, the group I ribozymes. However, group I introns have a lot more to offer than their own elimination by splicing. Intron RNA can circularize in at least three different ways and introns are mobile both at the DNA and RNA level. Some group I introns have a very complex organization incorporating functional genes and other sequence elements and have established deep relationships with their host genomes. Finally, group I introns can develop into new ribozymes with new biological functions.

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 3Ј 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 5Ј end, as well as truncation and polyadenylation downstream of the ORF 3Ј 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.