Functional characterization of X. laevis U5 snRNA genes.

Xenopus laevis U5 snRNA genes are found in several genomic arrangements, represented by a predominant tandem repeat of 583 bp and other minor repeats. Several copies of the major tandem repeat have been cloned and expressed in Xenopus oocytes. The transcripts assemble into U5 snRNPs which are recognized by anti-Sm antibodies. We have identified functional elements in the U5 gene promoter. Although similar in organization to other U snRNA gene promoters, U5 contains significant differences and is more efficiently expressed than the Xenopus U2 gene in oocytes. The proximal sequence element (PSE), although homologous to a mammalian consensus for this region (Skuzeski et al., 1984), does not resemble the previously characterized Xenopus U1 and U2 PSEs closely in sequence. The ATGCAAAT (octamer) part of the distal sequence element (DSE 1) is found in U5 in the orientation opposite to that in U1 and U2 gene promoters. DNase I protection experiments led to the identification of a third element (DSE 2), situated close to the octamer motif. Analysis of deletion mutants showed that both DSE 1 and 2 are essential parts of the U5 gene enhancer, and provides evidence that U snRNA enhancers are complex structures consisting of more than one site of DNA-factor interaction.