??? RNA Processing

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??? RNA Processing
In both prokaryotes and eukaryotes, primary RNA
transcripts undergo various alteration or
processing events to become mature RNAs The
three commonest types are 1, nucleotide
removal by nucleases 2, nucleotide
addition to the 5- or 3-end, 3,
nucleotide modification on the base or the sugar.
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??? rRNA processing and ribosomes
??rRNA processing in prokaryotes 1? Seven
different operons for rRNA dispersed thought out
the genome. 2?The primary transcript is
30S (6000nt). 3? Each contains one copy
of 5S,16S and 23S rRNA, together with some tRNA
sequence. 4? Rnase III is involved in rRNA
processing.
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30S transcript
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Primary rRNA processing in prokaryotes
Pre-16S rRNA
Pre-5S rRNA
Pre-t RNA
Pre-23S rRNA
Pre-tRNA
Primary transcript
RNase
Precursors
Mature rRNA
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5?The rRNA folds up into stem-loop structures by
base pairing in the transcript
16S rRNA?????
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6? Some specific proteins bind to the rRNA to
form RNP complex
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30S ???????
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??rRNA processing in eukaryotes
1?In the nucleolus, RNA polymerase I transcribes
the rRNA, which usually exist in tandem repeats
containing 100 or more copies of the
transcription units . 2? Primary transcript 47S,
containing
ETS external transcribed sequence
ITS internal
transcribed sequence
ETS
ITS
ITS
ETS
Pre-18S
Pre-5.8S
Pre-28S
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RNP(ribonucleoproteins)RNA-Protein complexes
3? The precursor folds and complexes with
proteins as it is being transcribed in the
nucleolus. 4?Many specific ribose methylations
take place at over 100 sites and is carried out
directly by a subset of snRNP particles. 5?snRNPs
contain snRNAs that have short streches of
complementarity to parts of the rRNA and, by base
pairing with it, they define where methylation
takes place.
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?????????????,????????????3-4?rRNA???
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??? tRNA Processing,
RNaseP and ribozymes
??tRNA processing
1?In prokaryotes, mature tRNA are generated by
processing longer pre-tRNA transcripts by RNases
D?E?F and P, followed by base modification which
are unique to each particular tRNA type.
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2? In eukaryotes, pre-tRNAs are synthesized with
an intron as well as extra 5 and 3 nucleotides
which are all removed during processing. The
3-terminal CCA is added by transferase. Many
base modification also occur.
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??RNaseP ????? P
1? RNaseP is endonuclease composed of one RNA
molecules and one protein molecules. It is a
simple RNP. 2 RNaseP are found both in
prokaryotes and eukaryotes. 3 The RNA component
alone works as an endonuclease. This RNA is a
catalytic RNA, or ribozyme.
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??Ribozyme
1 Definition Ribozyme is a catalytic RNA
molecules that catalyze particular biochemical
reactions in the absence of protein.
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2 Examples The self-splicing of the large
subunit of rRNA in Tetrahymena
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The concatenmeric molecules of the RNA genome of
plant viruses are able to fold up to self-cleave
themselves into monomeric genome-sized length.
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3 Application The minimum sequences
needed for self-cleaving are identified and
applied to inhibit gene expression by cleaving
mRNA molecules. It may be possible to
prevent virus
replication,
kill cancer cells,
discover the function of new genes by
inactivating them.
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