STIMULATION OF THE MOUSE RIBOSOMAL-RNA GENE PROMOTER BY A DISTAL SPACER PROMOTER

We show that the mouse ribosomal DNA (rDNA) spacer promoter acts in vi vo to stimulate transcription from a downstream rRNA gene promoter. Th is augmentation of mammalian RNA polymerase I transcription is observe d in transient-transfection experiments with three different rodent ce ll lines, under noncompetitive as well as competitive transcription co nditions, over a wide range of template concentrations, whether or not the enhancer repeats alone stimulate or repress expression from the d ownstream gene promoter. Stimulation of gene promoter transcription by the spacer promoter requires the rDNA enhancer sequences to be presen t between the spacer promoter and gene promoter and to be oriented as in native rDNA. Stimulation also requires that the spacer promoter be oriented toward the enhancer and gene promoter. However, stimulation d oes not correlate with transcription from the spacer promoter because the level of stimulation is not altered by either insertion of a funct ional mouse RNA polymerase I transcriptional terminator between the sp acer promoter and enhancer or replacement with a much more active hete rologous polymerase I promoter. Further analysis with a series of muta ted spacer promoters indicates that the stimulatory activity does not reside in the major promoter domains but requires the central region o f the promoter that has been correlated with enhancer responsiveness i n vivo.