CATALYTIC ACTIVITY OF AN RNA DOMAIN DERIVED FROM THE U6-U4 RNA COMPLEX

U6 RNA contains two regions that are essential for proper splicing of nuclear precursor messenger RNA (pre-mRNA). A comparison of putative s econdary structures of the U6-U4 RNA complexes from different phyla re vealed a conserved domain that is similar to the catalytic hammerhead RNA motif. Although no catalytic activity was detected in the mammalia n U6-U4 RNA complexes, two nucleotide changes in U6 RNA and one in U4 RNA conferred cleavage activity to the complex. Furthermore, the highl y conserved domain of the wild-type complex, without the accompanying flanking regions, cleaved an RNA substrate and exhibited other charact eristics of the hammerhead ribozyme. The possible involvement of this structure in pre-mRNA splicing is also discussed.