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Spacer sequences downstream of the 28S RNA coding region are part of the mouse rDNA transcription unit.

Evidence is presented that more than 300 bp of spacer sequences downstream of the 28S RNA coding sequence are part of the mouse rDNA transcription unit. Studies in two cell-free transcription systems as well as analysis of cellular RNA indicate that RNA polymerase I does not terminate within the 334 bp 3' terminal spacer sequences contained in the rDNA clone used. Quantitative hybridization data, S1 mapping experiments and Northern analysis of nuclear RNA showed that the 14 kb pre-rRNA molecules hybridize with the same efficiency to both the 28S and the 3' NTS specific DNA probe. This indicates that the rRNA precursor contains both at the 5' and 3' end several hundreds bases of external transcribed spacer sequences which are eliminated in subsequent processing reactions.