A new main category of catalytic RNA (the GIR1 ribozyme) has been discovered and is under investigation in our laboratory. The group I-like ribozyme (GIR1; approx. 200 nt) is a unique example of a naturally occurring ribozyme with an evolved biological fu nction. GIR1 generates the 5'-end of a nucleolar encoded endonuclease mRNA involved in intron mobility. Two natural sources of GIR1 are known, both found as internal processing ribozymes within two different introns (twin-ribozyme group I introns) in dist antly related organisms. The Naegleria GIR1 (NaGIR1) and Didymium GIR1 (DiGIR1) share fundamental features in ribozyme reaction and structural organization, but possess important differences compared to canonical group I intron ribozymes. The DiGIR1 riboz yme catalyses cleavage by a transesterification reaction with the joining of the first and the third nucleotide of the endonuclease mRNA resulting in a 2´, 5´ lariat cap. The basic regulatory mechanisms by which GIR1 participates in eukaryotic gene expres sion are still poorly understood, but investigations so far have discovered a direct link between RNA structure dynamics and ribozyme function. In order to progress towards our goal in revealing the biological role of GIR, this project will focus on chara cterization of GIR1 structure, catalysis, and tertiary RNA-RNA interactions important for regulation of protein gene expression.