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The U7 small nuclear RNA genes of Xenopus borealis.

Small nuclear RNA genes transcribed by RNA polymerase I1 have 3 conserved sequence elements necessary for efficient transcription. The distal sequence element, DSE (-250 to -200) is an enhancer containing at least one octamer sequence motif. The DSE has been shown to function independent of orientation and to some extent position [ I ] . The proximal sequence element, PSE (-60 to -50) is essential for the initiation of transcription and together with the 3' signal (a termination/processing sequence motif located between +8 and+ 15) form a coupled initiation/termination transcription system unique to snRNA genes [ 2 ] . We have isolated a 15kb genomic clone from Xenopus, which contains 14 'U7-like' sequences. Preliminary sequence analysis shows that these 'U7-like' sequences contain all of the sequence motifs expected of vertebrate snRNA genes, however the putative DSE, which contains 2 octamer motifs, between positions -67 and -86, is found only 4 base pairs upstream of the PSE. 8 of these 'U7-like' sequences were individually subcloned and expressed, by microinjection in Xenopus oocytes. 4 of the 8 genes were transcribed at high levels, producing a mature transcript of about 58 nucleotides in length. To investigate the compact nature of this gene, a 5' deletion series was constructed as shown in Fig. 18. These constructs were microinjected into Xenopus oocytes with32P-rGTP and a mouse U7 snRNA gene as an internal control. The results are shown in Fig. 1A. The initial conclusion drawn from these results is that deleting sequences upstream of AC (-90) has no detectable effect on the expression of the Xenopus U7 gene and that deleting sequences upstream of, and including, the PSE (AF) abolishes transcription. This result is constant with the comparative sequence data, as there are no conserved sequences upstream of -90 in any of the 14 U7 genes found on the 15kb clone. Thus the upstream signals required for expression of the Xenopus U7 gene lie within -52 and -90. Scintilation counts from the dried gel and scanning the autoradiograph with a Molecular Dynamics Scanning Densitometer. using the mouse U7 snRNA