Xenopus laevis U1 snRNA genes: characterisation of transcriptionally active genes reveals major and minor repeated gene families.

Xenopus laevis U1 snRNA genes are found in several different genomic arrangements. The major family of genes is organised in tandem repeats of 1.8 kb. The minor U1-family is much less abundant and is present on 1.2-kb HinfI restriction fragments. In addition there are genomic arrangements present in one or very few copies, which could represent the ends of repeating units. There is no evidence for the presence of U1 pseudogenes in Xenopus. A cluster of U1 snRNA genes consisting of a member of the minor class of U1 snRNA genes and two of the 'rarely represented' genes was cloned. All three genes were expressed upon microinjection into frog oocytes. A fragment containing 149 bp of 5' flanking sequence, the RNA coding sequence, and 27 bp of 3' flanking sequence was shown to be accurately transcribed into U1 snRNA. These oocyte transcripts are assembled into specific U1 snRNPs. Sequence comparison of the regions flanking Xenopus U1 and U2 snRNA genes showed the presence of two blocks of homology, which are also conserved in many other U snRNA genes. One of these blocks is found at position -60 to -50 before the coding sequence, and we discuss its possible role in the correct initiation of transcription. The other is 3' to the coding sequence and may be involved in the accurate production of mature 3' ends in the RNA.