Abstract
The Ribosomal DNA (rDNA) in mammals is organised into large clusters of tandem repeats each of which encodes a single 47S precursor for the 18S, 5.8S, and 28S ribosomal RNAs (rRNAs) that is flanked upstream and downstream by an Intergenic Spacer (IGS) originally referred to as the Non-Transcribed Spacer (NTS). However, in certain cells and under certain environmental conditions the IGS has been found to be transcribed at low level to generate a range of "Noncoding" RNAs (ncRNAs). These ncRNAs have been implicated in the regulation of rRNA synthesis, rDNA silencing and protein sequestration in response to environmental and oncogenic stresses and tumour suppression. Here we review data on the generation, regulation and potential functions of these ncRNAs. We suggest that the majority of the ncRNAs originate from a failure of RNA polymerase I transcription termination by the Reb1- and Myb-related transcriptional "road-block" factor TTF1 and link their expression with tumour suppression.