Post-transcriptional modification

Source: Wikipedia, the free encyclopedia.
(Redirected from
RNA processing
)

Transcriptional modification or co-transcriptional modification is a set of biological processes common to most eukaryotic cells by which an RNA primary transcript is chemically altered following transcription from a gene to produce a mature, functional RNA molecule that can then leave the nucleus and perform any of a variety of different functions in the cell.[1] There are many types of post-transcriptional modifications achieved through a diverse class of molecular mechanisms.

One example is the conversion of precursor

5' cap, the addition of a 3' polyadenylated tail, and RNA splicing. Such processing is vital for the correct translation of eukaryotic genomes because the initial precursor mRNA produced by transcription often contains both exons (coding sequences) and introns (non-coding sequences); splicing removes the introns and links the exons directly, while the cap and tail facilitate the transport of the mRNA to a ribosome and protect it from molecular degradation.[2]

Post-transcriptional modifications may also occur during the processing of other transcripts which ultimately become transfer RNA, ribosomal RNA, or any of the other types of RNA used by the cell.

mRNA processing

The image above contains clickable links
The structure of a
3' untranslated regions (blue) regulate translation into the final protein product.[3]
Prokaryote gene structure diagram
Operator
5'UTR
3'UTR
Transcription
RBS
RBS
Protein coding region
Protein coding region
mRNA
Translation
The image above contains clickable links
The structure of a
transcription of the gene into an mRNA. The mRNA untranslated regions (blue) regulate translation into the final protein products.[3]


5' processing

Capping

Capping of the pre-mRNA involves the addition of

S-adenosyl methionine to the guanine ring.[4] This type of cap, with just the (m7G) in position is called a cap 0 structure. The ribose of the adjacent nucleotide
may also be methylated to give a cap 1. Methylation of nucleotides downstream of the RNA molecule produce cap 2, cap 3 structures and so on. In these cases the methyl groups are added to the 2' OH groups of the ribose sugar. The cap protects the 5' end of the primary RNA transcript from attack by ribonucleases that have specificity to the 3'5' phosphodiester bonds.[5]

3' processing

Cleavage and polyadenylation

The pre-mRNA processing at the 3' end of the RNA molecule involves cleavage of its 3' end and then the addition of about 250

poly(A) tail. The cleavage and adenylation reactions occur primarily if a polyadenylation signal sequence
(5'- AAUAAA-3') is located near the 3' end of the pre-mRNA molecule, which is followed by another sequence, which is usually (5'-CA-3') and is the site of cleavage. A GU-rich sequence is also usually present further downstream on the pre-mRNA molecule. More recently, it has been demonstrated that alternate signal sequences such as UGUA upstream off the cleavage site can also direct cleavage and polyadenylation in the absence of the AAUAAA signal. It is important to understand that these two signals are not mutually independent and often coexist. After the synthesis of the sequence elements, several multi-subunit
Polyadenylate Polymerase (PAP). This complex cleaves the RNA between the polyadenylation sequence and the GU-rich sequence at the cleavage site marked by the (5'-CA-3') sequences. Poly(A) polymerase then adds about 200 adenine units to the new 3' end of the RNA molecule using ATP as a precursor. As the poly(A) tail is synthesized, it binds multiple copies of poly(A)-binding protein, which protects the 3'end from ribonuclease digestion by enzymes including the CCR4-Not complex.[5]

Introns Splicing

RNA splicing is the process by which

protein sequences. This process is known as alternative splicing
, and allows production of a large variety of proteins from a limited amount of DNA.

Histone mRNA processing

Histones H2A, H2B, H3 and H4 form the core of a

Cleavage and polyadenylation specificity factor 73 cuts mRNA between stem-loop and HDE[8]

Histone variants, such as

H2A.Z or H3.3, however, have introns and are processed as normal mRNAs including splicing and polyadenylation.[8]

See also

References

  1. PMID 11447102
    .
  2. ^ Berg, Tymoczko & Stryer 2007, p. 836
  3. ^
    ISSN 2002-4436
    .
  4. .
  5. ^ a b Hames & Hooper 2006, p. 221
  6. .
  7. .
  8. ^ .

Further reading