Ribosomal RNAs are generally synthesized as long, primary transcripts that
must be extensively processed to generate the mature, functional species. i
n Escherichia coil, it is known that the initial 30S precursor is cleaved d
uring its synthesis by the endonuclease RNase III to generate precursors to
the 16S, 23S, and 5S rRNAs. However, despite extensive study, the processe
s by which these intermediate products are converted to their mature forms
are poorly understood. In this article, we describe the maturation of 23S r
RNA. Based on Northern analysis of RNA isolated from a variety of mutant st
rains lacking one or multiple ribonucleases, we show that maturation of the
3' terminus requires the action of RNase T, an enzyme previously implicate
d in the end turnover of tRNA and in the maturation of small, stable RNAs.
Although other exoribonucleases can participate in shortening the 3' end of
the initial RNase III cleavage product, RNase T is required for removal of
the last few residues. In the absence of RNase T, 23S rRNA products with e
xtra 3' residues accumulate and are incorporated into ribosomes, with only
small effects on cell growth. Purified RNase T accurately and efficiently c
onverts these immature ribosomes to their mature forms in vitro, whereas fr
ee RNA is processed relatively poorly, in vivo, the processing defect at th
e 3' end has no effect on 5' maturation, indicating that the latter process
proceeds independently. We also find that a portion of the 23S rRNA that a
ccumulates in many RNase T- cells becomes polyadenylated because of the act
ion of poly(A) polymerase I. The requirement for RNase T in 23S rRNA matura
tion is discussed in relation to a model in which only this enzyme, among t
he eight exoribonucleases present in E. coil, is able to efficiently remove
nucleotides close to the double-stranded stem generated by the pairing of
the 5' and 3' termini of most stable RNAs.