The rRNA genes in most eucaryotic organisms are present in a tandem array. There is substantial evidence that transcription of one of these genes may not be independent of transcription of others. In particular, in the yeast Saccharomyces cerevisiae, the enhancer of rRNA transcription that lies 2.2 kilobases 5' of the transcription initiation site is at least partly within the upstream transcription unit. The transcription by RNA polymerase II of a wide variety of genes is profoundly influenced by cis-acting short-nucleotide sequences called enhancer elements that can lie upstream or downstream of the transcription unit, at a distance of up to several kilobases. In many cases, the enhancer elements bind protein factors that are necessary for enhancer function. Although a number of models have been proposed (reviewed in reference 30), a clear understanding of enhancer function is not at hand.Two features distinguish rRNA genes from other genes in the eucaryotic cell. They are transcribed by a specific enzyme, RNA polymerase I, and they are arranged in a tandem array. This unique genetic structure has led to speculation that transcription of an individual gene is not independent of transcription of the neighboring genes; e.g., termination at one transcription unit may lead directly to initiation at the neighboring downstream promoter (7,12,15, 26).We have identified a sequence element, specific for RNA polymerase I, that has many of the characteristics of an RNA polymerase II enhancer (9, 10). In this paper, we explore aspects of the function of RNA polymerase I and its enhancer to ask about the parallels between polymerase I and polymerase II enhancers and about the importance of the tandemness of rRNA genes to polymerase I activity.Transcription by RNA polymerase I has been reviewed recently (38). The spacers between individual rRNA transcription units vary in length from 2 kilobase pairs (kb) (yeasts) to >30 kb (mammals). Within them are found a variety of sequence elements that influence the level of transcription. In Xenopus laevis, for example, these include several copies of a sequence closely related to the promoter, dubbed spacer promoters (6,7,22,23,29,33), each followed by about 10 copies of a 60-to 81-base-pair (bp) repeat, that behave in some respects like enhancer elements (22,23,29). The spacer promoters are curious because although they can give rise to transcripts and can stimulate transcription from an adjacent real promoter (6,7,29), the transcription from the spacer promoter seems not to be involved in this stimulation (7,25). Repetitive elements, some related to the promoter, have also been found in the nontranscribed spacer * Corresponding author. t Present address: