Site-directed mutational analysis of a U4 small nuclear RNA gene proximal sequence element. Localization and...

The genes that encode the small nuclear RNAs (snRNAs) are unusual RNA polymerase II transcription units in that 5'-flanking DNA sequences more than 50 base pairs upstream of snRNA genes are essential for specifying the transcription initiation site. The relevant cis-acting DNA sequence, termed the proximal sequence element (PSE), is required for both transcription initiation and 3'-end formation of snRNAs. We have used site-directed mutagenesis and expression in Xenopus oocytes to map nucleotides important for the function of the chicken U4B snRNA gene PSE. The results indicate that nucleotide sequences upstream of position -65 are not required for U4B PSE activity. However, nucleotides lying within a region 53-65 base pairs upstream of the U4B gene are essential for obtaining a detectable level of U4B gene expression. Six nucleotides between positions -53 and -59 were identified at which base substitutions reduced the transcriptional activity of the U4B gene.