A nuclear protein fraction binding to dA/dT-rich sequences upstream from the radish rDNA promoter [corrected].

The external spacer (ES) of rRNA nuclear genes (rDNA) contains the sequences that control rDNA transcription initiation and enhancement. The ES is also characterized in most species by the presence of multiple repeated elements. In higher plants very few data are available on the cis- and trans-acting elements which control rDNA transcription. Using electrophoretic mobility shift assays (EMSA) it is shown that nuclear extracts from young radish leaves (NER) contain a protein fraction which binds to specific sequences in the radish ES. DNase I footprinting analysis allows mapping of the NER protein binding to dA/dT homopolymer stretches and to a 13-bp dA/dT-rich short repeat, found both in the seven approximately 100 bp repeat regions (located -1077 to -740 from transcription initiation site) and the region (-120 to -55) containing the putative promoter for rDNA transcription initiation. Whether this ES binding is due to a single or several different proteins is not known. So far, protein(s) binding to dA/dT-rich regions of a plant rDNA ES has not yet been described. Whether it is a specific RNA polymerase I transcription factor(s) or plays a more general role in genome expression remains to be elucidated.