A component of Drosophila RNA polymerase I promoter lies within the rRNA transcription unit

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Abstract
Genes transcribed by RNA polymerase II have been characterized as having several control regions upstream of the site at which transcription is initiated, and the 'CAAT' and 'TAATA' regulatory sequences are conserved in content and location in diverse eukaryotes. The transcription of 5S RNA and tRNA by polymerase III is regulated by promoters that are actually within the genes; these are also conserved in diverse eukaryotes. In addition, sequences upstream of tRNA genes can affect RNA synthesis. Recently, it has been determined that DNA sequences influencing the expression of the 18S and 28S ribosomal RNA genes (rDNA) lie upstream of the site of transcription initiation. We have shown that a major component of the promoter of Drosophila rRNA polymerase I activity involves the region -43 to -27, where initiation is at +1. Here we present evidence that another major component of the polymerase I promoter lies within the first four nucleotides of the external transcribed spacer (ETS). Drosophila polymerase I control signals, therefore, lie both upstream of and within the rRNA transcription unit.