A transcription terminator located upstream of the mouse rDNA initiation site affects rRNA synthesis

Mouse ribosomal genes have a short sequence upstream of the transcription initiation site that is related in structure and function to the terminator boxes previously identified at the 3′ end of the transcription unit. This upstream terminator recognizes the same protein factor as the 3′-terminal sites and is able to terminate RNA polymerase I transcription in vitro. S1 mapping and nucleolar run-on experiments reveal the presence of 5′-terminal spacer transcripts that are terminated at this site. These transcripts probably derive from spacer promoters, one of which has been identified approximately 2 kb upstream of the pre-rRNA start site. The interaction of a specific nuclear factor with the upstream terminator increases the efficiency of initiation, suggesting that transcription termination and initiation at the adjacent promoter work in an interrelated fashion.