24 RNA SPLICING AND PROCESSING (Full Edition)
21 Production of rRNA requires cleavage events
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45S RNA is a precursor that contains the sequences of both major ribosomal RNAs (28S and 18S rRNAs).
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The large and small rRNAs are released by cleavage from a common precursor RNA.
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The major rRNAs are synthesized as part
of a single primary transcript that is processed to generate the mature
products. The precursor contains the sequences of the 18S, 5.8S, and
28S rRNAs. In higher eukaryotes, the precursor is named for its
sedimentation rate as 45S RNA. In lower eukaryotes, it is smaller (35S in yeast).
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Figure 24.37
Mature eukaryotic rRNAs are generated by cleavage and trimming events from a primary transcript.
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The mature rRNAs are released from the precursor by
a combination of cleavage events and trimming reactions
(for review see 980). Figure 24.37
shows the general pathway in yeast. There can be variations in the
order of events, but basically similar reactions are involved in all
eukaryotes. Most of the 5' ends are generated directly by a cleavage
event. Most of the 3' ends are generated by cleavage followed by a 3' –
5' trimming reaction.
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Many ribonucleases have been implicated
in processing rRNA, including the exosome, an assembly of several
exonucleases that also participates in mRNA degradation
(see mRNA degradation involves multiple activities).
Mutations in individual enzymes usually do not prevent processing,
suggesting that their activities are redundant and that different
combinations of cleavages can be used to generate the mature molecules.
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There are always multiple copies of the transcription unit for the rRNAs.
The copies are organized as tandem repeats
(see The repeated genes for rRNA maintain constant sequence).
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5S RNA is transcribed from separate genes
by RNA polymerase III. Usually the 5S genes are clustered, but are
separate from the genes for the major rRNAs. (In the case of yeast, a
5S gene is associated with each major transcription unit, but is
transcribed independently.)
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Figure 24.38
The rrn operons in E. coli contain
genes for both rRNA and tRNA. The exact lengths of the transcripts
depend on which promoters (P) and terminators (t) are used. Each RNA
product must be released from the transcript by cuts on either side.
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There is a difference in the
organization of the precursor in bacteria. The sequence corresponding
to 5.8S rRNA forms the 5' end of the large (23S) rRNA, that is, there
is no processing between these sequences. Figure 24.38 shows that the
precursor also contains the 5S rRNA and one or two tRNAs. In E. coli,
the 7 rrn operons are dispersed around the genome; four rrn
loci contain one tRNA gene between the 16S and 23S rRNA sequences, and the other rrn
loci contain two tRNA genes in this region. Additional tRNA genes may
or may not be present between the 5S sequence and the 3' end. So the
processing reactions required to release the products depend on the
content of the particular rrn locus.
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In both prokaryotic and eukaryotic rRNA
processing, ribosomal proteins (and possibly also other proteins) bind
to the precursor, so that the substrate for processing is not the free
RNA but is a ribonucleoprotein complex.
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980 Venema, J. and Tollervey, D.
(1999).
Ribosome synthesis in S. cerevisiae.
Annu. Rev. Genet. 33, 261-311.
PubMed Journal
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©Jones and Bartlett Publishers (2007)
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