Sequence differences upstream of the promoters are involved in the differential expression of the Xenopus somatic and oocyte 5S RNA genes.

The Xenopus somatic and oocyte 5S RNA genes are differentially expressed in extracts of whole oocytes. In such extracts, sequence differences preceding the internal promoters significantly alter the relative activities of these genes. Following exchange of the sequences preceding the promoter, the activity of the somatic 5S gene decreased and that of the oocyte 5S gene increased. As a result, a 100 fold somatic transcriptional advantage was reduced to 5 fold. Analysis of deletion mutants showed that the relevant sequence differences are located between -34 and +37 relative to the initiation site. The observed transcriptional modulation is due both to sequence differences 5' to the initiation site and at positions 30 and 37 within the coding region.