Proteins that bind to the yeast rDNA enhancer.

The transcription of ribosomal RNA genes differs from that of other genes in several respects: the use of a specialized polymerase, the generally high level of transcription, and the tandem arrangement of the genes. In the yeast Saccharomyces cerevisiae, we identified a nucleotide sequence in the "nontranscribed" spacer region that had many characteristics of an enhancer of transcription (8). More recently, it has become apparent that transcription of this sequence occurs (9) and that it may also be involved in some aspect of termination of 35 S rRNA transcription. The likelihood that there are protein factors involved in termination and activation of transcription and that these may participate in the coupling of the transcription of adjacent rRNA genes led us to search for proteins that might bind to the enhancer. We have identified two such proteins, termed REB1 and REB2, that bind to the enhancer and protect specific sequences from attack by chemical and enzymatic reagents. It is noteworthy that there is a second REB1 binding site approximately 210 base pairs upstream of the origin of transcription of rRNA and that binding of REB1 to this site alters the conformation of DNA adjacent to the site of initiation.