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 ha s an unusual twin-ribozyme organization, One of the ribozymes (DiGIR2) cata lyses intron excision and exon ligation. The other ribozyme (DiGIR1), which along with the endonuclease-encoding I-DirI open reading frame (ORF) is in serted in DiGIR2, carries out hydrolysis at internal processing sites (IPS1 and IPS2) located at its 3' end. Examination of the in vivo expression of D1SSU1 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 ORE 3' end. A spli ceosomal intron, the first to be reported within a group I intron and the r DNA, is removed before the I-DirI mRNA associates with the polysomes, Taken together, our results imply that DiSSU1 uses a unique combination of intro n-supplied ribozyme activity and adaptation to the general RNA polymerase I I pathway of mRNA expression to allow a protein to be produced from the RNA polymerase I-transcribed rDNA.