The 30 S ribosomal precursor RNA from Escherichia coli. A primary transcript containing 23 S, 16 S, and 5 S sequences.

The 30 S ribosomal precursor RNA has been prepared from Escherichia coli AB301/105 (RNase III-) labeled with 32-PO4 in the presence of chloramphenicol. Direct nucleotide sequence studies yield the following information. 1. The major 5'-terminal sequence in our precursor preparations is pppA-C-U-G-. 2. Treatment of the precursor RNA with purified ribonuclease III in vitro releases species sedimenting near 23 S and 17 S, neither of which retain the pppA- end, plus a collection of small fragments with chain lengths of less than 400 nucleotides. 3. The RNase III product sedimenting near 17 S (16 SIII) appears identical with the 17 S RNA typically isolated from pulse-labeled or chloramphenicol-treated cells or from several mutants deficient in ribosome assembly: fingerprint analysis reveals the presence of the same additional RNase T1 oligonucleotides and the 5' terminus (pU-G-) previously described for 17 S RNA. A 3'-terminal T1 oligonucleotide (which was not previously identifiable in the case of the 17 S precursor) has been isolated from 16 S III and its sequence determined: C-U-C-A-C-A-C-A. 4. 5 S rRNA sequences are contained in an RNase III-released fragment of approximately 300 nucleotides. This molecule lacks the 5' terminus of the mature 5 S RNA. The implications of these findings with respect to the control of ribosomal RNA synthesis, the pathways of rRNA processing in vivo, and the specificity of RNase III cleavage of natural substrates are discussed.