Spacer transfer RNAs in ribosomal RNA transcripts of E. coli: processing of 30S ribosomal RNA in vitro

At least three different transfer RNAs are produced by in vitro processing of 30S ribosomal RNA which accumulates in RNAase III- strains of E. coli. Two of these tRNAs, tRNAGlu2 and tRNAIle1, have previously been shown to be "spacer tRNAs"--that is, genes for their synthesis are located in rRNA transcription units between the cistrons for 16S and 23S rRNAs (Lund et al., 1976). The third tRNA whose sequences are contained in 30S rRNA is tRNAAla1B. In addition to the tRNAs, 5S rRNA and several other 4S fragments are produced. Some of these 4S fragments may represent additional spacer tRNAs. One fragment, about 70 nucleotides long, arises from the 5' end of the 17S precursor of 16S rRNA. Four or five other tRNAs are hydrogen-bonded to 30S rRNA as we prepare it; one or more of these tRNAs may also be a spacer tRNA. The enzymes that process tRNAs out of 30S rRNA are associated with ribosomes, but can be removed from them by washing in 0.2 M NH4Cl; the enzymes required for 5S rRNA processing remain bound to the 0.2 M NH4Cl-washed ribosomes. Treatment of 30S rRNA with purified RNAase III produces 6-8S fragments which contain the sequences of tRNAGlu2, tRNAAla1B and 5S rRNA.