Linker scanner mutagenesis of the Xenopus laevis ribosomal gene promoter

We have assayed a series of linker scanner mutants which cover the Xenopus laevis ribosonal gene promoter at approximately ten base pair intervals. All of these mutations adversely affect promoter activity with the exception of one mutation vhich stimulates activity. Thus, none are neutral. We show that most of the mutations can be partially rescued by ligating a block of enhancer elements upstream of the promoter. In addition, we have made extracts from liver nuclei which produce DNasel protection footprints over the promoter. Analysis of both strands reveals a prominent footprinting donain from about-5 to -30. However, lesser changes in the digestion pattern are detected over most of the promoter. Previously published analyses have suggested that this pronoter might be conposed of three functional domains. The experiments presented here suggest that either 1) the three putative domains are so closely arranged that the boundaries are difficult to discern, or 2) the situation is more complex.