High resolution studies of the Xenopus laevis ribosomal gene promoter in vivo and in vitro

The first high resolution maps of the Xenopus laevis ribosomal promoter and its flanking regions (-179 to +14) have been created by assaying point mutants both in oocyte and in vitro. Within the promoter boundaries (-141(-145) to +3(+4)), domains analogous to the Core Promoter and "Upstream Control Element" (UCE) were clearly detected. The base pairs at -133, within the UCE, and -20, -10, -7, and +3, within the Core, were all shown to be especially important for promotion. Between the Core and UCE, two central promoter elements (CPEs) were also resolved. Surprisingly, these CPEs did not correspond to the highly conserved enhancer homology (approximately -70 to -110), but to CCCGGCC motifs immediately flanking it. Although xUBF was shown to be a limiting component for in vitro transcription, none of the point mutations was found to affect the interaction of this factor with the promoter.

Readthrough enhancement and promoter occlusion on the ribosomal genes of Xenopus laevis

An RNA polymerase I termination site is found just upstream of the ribosomal gene promoter in mammals and amphibia. It has been suggested that this termination site may actively enhance ribosomal transcription in a process known as readthrough enhancement or that it may simply prevent the disruption of initiation complexes or promoter occlusion. There is, however, a consensus of opinion that the terminator is important for efficient ribosomal transcription. Here we have quantitatively investigated the relative importance of readthrough enhancement and promoter occlusion on the transcription of the microinjected Xenopus laevis ribosomal gene. The results show that, in this system, promoter occlusion is limited and terminator mutations predominantly affect readthrough enhancement. The terminator is shown to be unnecessary for the enhancer activity of the rest of the ribosomal spacer. Model calculations suggest that readthrough enhancement could be explained by polymerase recycling and that it may be unnecessary to postulate a specific mechanism of polymerase handover.