This is the published version. Copyright 1980 by the Oxford University Press. All rights reserved.Escherichia coli cells lacking the ribosomal RNA processing enzyme RNase III do not excise the normal RNA precursors pl6a (17S) and p23a from nascent rRNA transcripts. These cells produce, instead, slightly larger pl6b and p23b precursors. Digestion of pl6b or p23b rRNA with RNases A plus Tl yields double-stranded fragments composed of sequences located at both the 5′ and the 3′ end regions of the molecules. The terminal duplex, or stem, of p 16b contains sequences surrounding the site of RNase III processing which in wild-type cells produces pl6a rRNA: the p23b stem likewise contains an intact RNase III cleavage site. The results confirm our earlier prediction for the structure of rRNA transcripts, and also yield a definite secondary structure for the pl6 stem, which was not uniquely determined by the corresponding DNA sequence. These experiments demonstrate the absence of significant RNase III processing activity in rnc-105 strains of E. coli, and implicate the participation of another endonuclease(s) 1n rRNA processing in mutant and wild-type cells
