Rapid initial cleavage of nascent pre-rRNA transcripts in yeast.

In yeast cells, as in many other eukaryotes, the initial step in the processing of the pre-rRNA primary transcript is removal of external transcribed spacer (ETS) sequences from the 5' end of the transcript. We show here, both by Northern analysis and by quantitative hybridization procedures using cloned yeast ETS sequences, that in cells growing exponentially at 23 degrees C most nascent pre-rRNA transcripts no longer contain ETS sequences. Moreover, quantitative hybridization shows that uncleaved pre-rRNA molecules that still contain ETS sequences have a half-life of only 0.5 minute, a value that supports the finding that ETS removal usually takes place before pre-rRNA transcription is complete. Under these same conditions, the half-life of ETS sequences is shown to be only 1.0 minute.