Differential expression of multiple U1 small nuclear RNAs in oocytes and embryos of Xenopus laevis

The small nuclear RNA, U1, is a highly conserved, 165 nucleotide long RNA which has been implicated in the processing of mRNA precursors. We present evidence that in the amphibian X. laevis there exist at least seven species of U1 RNA, which differ in sequence but not in length. Strikingly, these RNAs are not coordinately expressed. Two of the U1 RNAs are the predominant U1 species transcribed in the late blastula-early gastrula stages of Xenopus embryogenesis. These two RNAs, designated xU1a and xU1b, are not synthesized in significant amounts in stage 6 oocytes; a different set of U1 RNAs are expressed during late oogenesis. In a Xenopus cultured cell line, all of the U1 RNA species are expressed. Possible functions and developmental significance of these multiple U1 RNA species are discussed.