A Hela cell S-100 extract primed with a purified human rDNA containing clone, has been shown to be capable of initiating specific α-amanitin-resistent RNA transcripts. By using a number of truncated templates, the site of RNA polymerase I initiation in vitro has been identified. The origin of transcription in vitro and in vivo was further defined by S1-mapping studies with total Hela cell RNA or RNA isolated from the in vitro transcription reaction. The initiation site was found to be the same. The nucleotide sequence of an 848 bp region around the initiation site, has also been determined. A perfect 15 bp homology has been found to exist between human and mouse rDNA very close to the origin of transcription, although little homology exists elsewhere. Sequences homolgous to the origin of transcription region were not found repeated within a 12 kb non-transcribed spacer segment upstream from it.
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