Processing of the external transcribed spacer of murine rRNA and site of action of actinomycin D.

The primary rRNA transcript contains a large external transcribed spacer (ETS) approximately 4,000 nucleotides in length. We have used subcloned DNA probes derived from the 5' end of the ETS in conjunction with Northern blot analysis of murine nuclear RNA to examine processing of this region. In agreement with the results of previous investigators, we find that the large rRNA precursor lacks part of the ETS region. These ETS sequences are also missing from subsequent rRNA processing intermediates. Experiments using actinomycin D confirm that the excision of portion of the ETS is an early event in rRNA processing. In addition, in the presence of actinomycin D small RNA species accumulate which hybridize to a probe specific for the 5' end of the ETS. The length of these abbreviated transcripts defines a region of rDNA which is probably a target for this drug.