Sequence, organization and transcriptional analysis of a gene encoding a U1 snRNA from the axolotl, Ambystoma mexicanum.

AmU1, a DNA fragment containing a U1 small nuclear RNA (snRNA)-encoding gene, was isolated from the axolotl, Ambystoma mexicanum. Although this U1 snRNA, produced in axolotl oocytes, exhibits the lowest degree of sequence conservation among vertebrates, its secondary structure is maintained by a number of compensatory base changes. The proximal sequence element (PSE) is only weakly similar to that of the previously characterized Xenopus laevis PSE. Exchanging either the entire upstream regions with their X. laevis U1 (XlU1) homologues or only the PSE with the XlU1 PSE increases the transcription rate of the AmU1 gene to a level similar to that of the XlU1 gene. However, while allowing the AmU1 gene to be transcribed with high efficiency in X. laevis oocytes, the strict swapping of the 12-bp constituting the XlU1 PSE does not confer competitive ability to the AmU1 gene. We present evidence that the PSE is the major, but not the only element responsible for the low template activity of the AmU1 gene in X. laevis oocytes and our data suggest that other sequences, perhaps flanking the PSE, might also influence the binding of factor(s) participating in the assembly of the transcription complex.