Xenopus laevis U2 snRNA genes: tandemly repeated transcription units sharing 5' and 3' flanking homology with other RNA polymerase II transcribed genes.

Xenopus laevis U2 small nuclear RNA (snRNA) genes were isolated and expressed by microinjection into frog oocytes. The genes are organised in short tandemly repeated units of approximately 830 bp. Some of the cloned tandem repeats are closely linked to genes coding for U5 snRNA, tRNA and an uncharacterised 7S RNA. No evidence was found for U2 snRNA pseudogenes. Single repeat units are transcriptionally active, showing that all the signals necessary for U2 snRNA transcription are included in an 831-bp segment of DNA. Sequence analysis of a cloned repeat unit showed that Xenopus and rat U2 snRNAs are 94% homologous. Flanking regions 5' and 3' to the coding sequence were found which shared extensive homology with similarly positioned sequences in human U1 snRNA genes. Part of the 3' non-coding region homology (consensus TTTNAAAGAAT) was found in many other genes transcribed by RNA polymerase II.