Transcription of spacer sequences in genes coding for ribosomal RNA in Xenopus cells.

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Abstract
Untreated X. laevis cells synthesize RNA molecules (5S-23S) containing sequences complementary to the external nontranscribed spacer of the genes coding for rRNA. In cells incubated in the cold, maturation cleavages of the rRNA precursors are inhibited. Concomitantly, the relative proportion of spacer transcripts detectable in newly synthesized RNA is increased and some sequences complementary to those of the nontranscribed spacer are found in the heavy shoulder of the 40S precursor rRNA peak. In such events, transcription initiation seems to take place within the spacer because its middle and right BamHI endonuclease fragments are preferentially transcribed; only few RNA sequences complementary to the left spacer fragment are found. At least some spacer regions contain promoters for transcription and can be transcribed either into a special class of spacer transcripts or into molecules covalently linked to rRNA precursor.