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> > >Unit 4: > > >Post Transcriptional Modifications, Processing of Eukaryotic RNA and
Gene Regulation > > >
Processing of tRNA and rRNA
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> > >Unit 4: > > >Post Transcriptional Modifications, Processing of Eukaryotic RNA and
Gene Regulation > > >
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>Processing of tRNA >
>The processing of transfer RNA (tRNA) involves several steps that are
necessary for the production of functional tRNA molecules. tRNAs are
synthesized as precursor molecules that require several modifications to
form mature tRNAs. The processing steps include: >
>Transcription: The DNA sequence that encodes tRNA is transcribed into a
primary transcript by RNA polymerase III. >
>5' end processing: The primary transcript of tRNA is cleaved by an
endonuclease to remove the 5' leader sequence, which is not part of the
mature tRNA. >
>3' end processing: The primary transcript of tRNA is cleaved by an
endonuclease to remove the 3' trailer sequence. A CCA sequence is then added
to the 3' end of the tRNA by the enzyme tRNA nucleotidyltransferase. >
>Base modification: Specific bases in the tRNA molecule are modified
enzymatically to produce mature tRNA. These modifications include the
conversion of uridine to pseudouridine, the methylation of adenosine,
cytosine, and guanine, and the thiolation of uridine. >
>Splicing: In some cases, the primary transcript of tRNA contains introns
that must be removed by splicing. This process is carried out by
endonucleases and ligases to produce mature tRNA. >
>Quality control: The processed tRNA molecules are subjected to a quality
control mechanism to ensure that they are functional and properly folded.
Misfolded or defective tRNA molecules are targeted for degradation by the
cell. >
>These processing steps are essential for the production of functional tRNA
molecules, which play a critical role in protein synthesis by transferring
amino acids to the ribosome during translation. >
>rRNA and tRNA are structural molecules that aid in protein synthesis, but
are not themselves translated into proteins. >
>Ribosomal RNA (rRNA) is the structural molecule that makes up more than
half the mass of the ribosome and aids in protein synthesis. >
>Transfer RNA (tRNA) recognizes codons on the mRNA and places the
appropriate amino acid in that position. >
>rRNA is processed from the large pre-rRNA by cleaving the large rRNA and
methylating some of its nucleotides. >
>tRNAs are processed from pre-tRNAs by truncating both ends of the
pre-tRNA, optionally adding CCA trinucleotides to the 3' ends, removing
existing introns, and chemically modifying an average of 12 nucleotides
per tRNA. increase. >
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>Processing of tRNAs and rRNAs >
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>tRNA and rRNA are structural molecules involved in protein synthesis.
However, these RNAs themselves are not translated. In eukaryotes, rRNA
precursors are transcribed, processed, and assembled into ribosomes in the
nucleolus, whereas pre-tRNAs are transcribed and processed in the nucleus
and released into the cytoplasm, where they bind free amino acids for
protein synthesis. will be >
>Ribosomal RNA (rRNA) >
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>The four eukaryotic rRNAs are first transcribed as two long precursor
molecules. One contains only pre-rRNA and is processed into 5S rRNA. The
other is 28S, 5.8S, and 18S rRNA. The enzyme then cleaves the precursor into
subunits corresponding to each rRNA. In bacteria, there are only three
rRNAs, all transcribed into long precursor molecules that are cleaved into
individual rRNAs. Some bases in pre-rRNA are methylated to increase
stability. Mature rRNA constitutes 50-60% of each ribosome. Some ribosomal
RNA molecules are purely structural, while others have catalytic or binding
activity. Eukaryotic ribosomes are composed of two subunits, the large
subunit (60S) and the small subunit (40S). The 60S subunit is composed of
28S rRNA, 5.8S rRNA, 5S rRNA and 50 proteins. The 40S subunit is composed of
18S rRNA and 33 proteins. Bacterial ribosomes consist of two similar
subunits that differ slightly in composition. The large bacterial subunit is
called the 50S subunit and consists of 23S rRNA, 5S rRNA and 31 proteins,
and the small bacterial subunit is called the 30S subunit and consists of
16S rRNA and 21 proteins. Two subunits combine to form a functional ribosome
capable of making proteins. >
>Transfer RNA (tRNA) >
>
Each tRNA binds to a specific amino acid and transfers it to the ribosome.
The mature tRNA adopts a three-dimensional structure through
intramolecular base pairing, placing an amino acid binding site at one end
and an anticodon in an unpaired nucleotide loop at the other end.
Anticodons are three-nucleotide sequences unique to each tRNA that
interact with messenger RNA (mRNA) codons through complementary base
pairing.
There is a different tRNA for every 21 amino acids. Most amino acids
can be carried by multiple tRNAs.
In all organisms, tRNA is transcribed in a pre-tRNA form that requires
several processing steps before the mature tRNA is available for
translation. In bacteria, multiple tRNAs are often transcribed as a single
RNA. The first step in their processing is to digest the RNA to release
individual pre-tRNAs. In archaea and eukaryotes, each pre-tRNA is
transcribed as a separate transcript.
>The process of converting pre-tRNA to mature tRNA involves five
steps. >
>The 5' end of the pre-tRNA, the so-called 5' leader sequence, is
cleaved. >
>The 3' end of the pre-tRNA is cleaved. >
>In all eukaryotic pre-tRNAs, a CCA nucleotide sequence is added to
the 3' end of the pre-tRNA after truncating the original 3' end, but
some bacterial and archaeal pre-tRNAs only included. Some bacterial
and archaeal pre-tRNAs already have a CCA encoded just before the 3'
cleavage site of the transcript, so no addition is necessary. The CCA
at the 3' end of the mature tRNA is where amino acids are added to the
tRNA. >
>Some nucleotides of the pre-tRNA are chemically modified to change
the nitrogenous base. On average, approximately 12 nucleotides are
modified per tRNA. The most common modifications are the conversion of
adenine (A) to pseudouridine (ψ), adenine to inosine (I), and uridine
to dihydrouridine (D). However, over 100 other changes can
occur. >
A significant number of eukaryotic and archaeal pre-tRNAs have introns
that require splicing. Introns are rare in bacterial pre-tRNAs, but are
occasionally present and spliced out. After processing, the mature
pre-tRNA is ready to bind its cognate amino acid. A tRNA's cognate amino
acid is that specified by its anticodon. This amino acid addition is
called tRNA charging. In eukaryotes, mature tRNAs are produced in the
nucleus and transported to the cytoplasm for loading.
>
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