Ribosomal RNAs are generally synthesized as long,
primary transcripts that must be extensively processed
to generate the mature, functional species. In Escherichia
coli, it is known that the initial 30S precursor is
cleaved during its synthesis by the endonuclease RNase
III to generate precursors to the 16S, 23S, and 5S rRNAs.
However, despite extensive study, the processes by which
these intermediate products are converted to their mature
forms are poorly understood. In this article, we describe
the maturation of 23S rRNA. Based on Northern analysis
of RNA isolated from a variety of mutant strains lacking
one or multiple ribonucleases, we show that maturation
of the 3′ terminus requires the action of RNase T,
an enzyme previously implicated 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 extra
3′ residues accumulate and are incorporated into
ribosomes, with only small effects on cell growth. Purified
RNase T accurately and efficiently converts these immature
ribosomes to their mature forms in vitro, whereas free
RNA is processed relatively poorly. In vivo, the processing
defect at the 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 accumulates
in many RNase T− cells becomes polyadenylated
because of the action of poly(A) polymerase I. The requirement
for RNase T in 23S rRNA maturation is discussed in relation
to a model in which only this enzyme, among the eight exoribonucleases
present in E. coli, 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.