The two embryonic U1 small nuclear RNAs of Xenopus laevis are encoded by a major family of tandemly repeated genes.

Abstract

We have identified a large family of Ul RNA genes in Xenopus laevis that encodes two distinct species of Ul RNA. These genes are expressed primarily at the onset of transcription in the 4,000-cell embryo

Human U1 small nuclear RNA genes: extensive conservation of flanking sequences suggests cycles of gene amplification and transposition.

“…The human U2 tandem array maps to a single site on chromosome 17 (28a). The vast majority of Ul genes in Xenopus laevis are arranged in a tandem array of 1.8-kb repeat units (32,66), and the Xenopus U2 genes lie within a separate tandem array with an 830-bp repeat unit (38). The genes encoding the Ul and U2 RNAs of sea urchins also are arranged in separate tandem arrays with repeat units of 1.4 and 1.1 kb, respectively (6,7).…”

Structure and expression of a second sea urchin U1 RNA gene repeat

“…In chickens the Ul genes are not tandemly repeated (8) and the rat Ula genes and mouse Ulb genes are also not present in a tandemly repeated unit (5,6). In Xenopus some of the Ul genes are present in tandemly repeated units (9,10). The Xenopus repeat units identified thus far each contain two Ul genes which code for Ul RNAs with different sequences (10).…”

“…The expression of the Ul genes in Xenopus is regulated during development (9,11). There is developmentally regulated expression of the mouse Ul Nucleic Acids Research RNAs as well (12).…”

Orientation-dependent transcriptional activator upstream of a human U2 snRNA gene.

“…The most telling mutation (dl-2231-218) removed just 4 bp in the middle of region III but reduced transcription to the same low level (2 to 5% of wild type) as deletion of all sequences upstream of position -62 (Fig. 4 and 7 homology (16 of 17 bp match) at a comparable position in the Xenopus and rat U2 genes (Fig. 1B).…”

“…The genes encoding U2 and Ul, the most abundant of the snRNAs, have been cloned from a number of vertebrates, including rats (38,41), mice (19,28), Xenopus laevis (3,16,20,46), and humans (18,26,39,45). The genomic organization of the human U2 genes is particularly simple.…”