The two embryonic U1 RNA genes of Xenopus laevis have both common and gene-specific transcription signals.

We have cloned and sequenced the 1842-bp repeat DNA encoding the two Xenopus laevis embryonic U1 RNAs, xU1a and xU1b. Although these two U1 RNAs are almost identical in sequence and are coordinately expressed during early embryogenesis, the flanking sequences of their genes show very little homology. Both genes contain two short conserved sequences, centered around positions -55 and +19, that probably are essential for 5' and 3' end formation of U1 RNAs, respectively. Efficient transcription of either gene in stage VI oocytes requires gene-specific promoter elements, located upstream of position -220. In the xU1b gene, these required 5'-flanking sequences include an 18-bp palindrome that has potential for Z-DNA formation. When injected separately into stage VI oocytes, the xU1a and xU1b genes are equally well transcribed, but co-injection of the two genes, either as the full length repeat or as two separate subclones, results in preferential accumulation of xU1b RNA. This competitive advantage of the xU1b gene in injected oocytes apparently is the result of preferred binding of one or more transcription factors that are limiting in these oocytes.