RNA Processing
I.
INTRODUCTION
A. Messenger RNA
1. The primary RNA transcript of mRNA is processed in the nucleus
a) Processing includes adding a cap to the 5' end, adding a poly-A tail to the 3'
end, and removing introns while connecting exons
B. Transfer and ribosomal RNA
1. Stability
a) Much more stable then mRNA
2. Neither are translated
3. Post transcriptional processing
a) Highly processed
b) Differences from primary RNA transcripts
(1) Ends in a 5'-monophosphate
(a) Rather then expected 5'-triphosphate
(2) All tRNAs contain bases other than U, C, G, and A
(a) Bases are modified after transcription
II.5'-CAP
STRUCTURE OF MESSENGER RNA
A. Structure
1. 7-methylguanosine (7mGTP) is added to the 5' end of primary transcripts
destined to become mRNA
a) 7mG is a methylated guanosine derivative
b) There is an unusual 5' to 5' bonding between 7mG and the 5' ribonucleotide
B. Mechanism
1. Catalyzed by guanylyl transferase
a) 7-MeGppp + pppApNpNp  GpppApNpNp + pp + p
C. Function
1. Protection from degradation by nucleases
2. Transport from the nucleus into the cytoplasm
III.3'-POLY
A TAIL OF MESSENGER RNA
A. Structure
1. 20 to up to 250 AMP are found on the 3' end of mRNA
2. These were added after transcription
a) They are not encoded by DNA
b) Catalyzed by a nuclear enzyme called poly(A) polymerase
3. An enzyme cleaves 10 - 53 bases from the 3' end at the highly conserved
AAUAAA sequence and then sequentially adds As
a) Mutants lacking the conserved AAUAAA sequence cannot add the poly-A
tail to their mRNA
B. Function
1. Limits degradation of mRNA
IV.INTRON
REMOVAL FROM MESSENGER RNA
A. Introduction
1. Primary RNA transcripts are much larger in eukaryotes than are the mRNAs
processed from them
2. The RNA transcript contains nucleotide sequences that are not used in
translation and are not found in mRNA molecules
a) These sequences are referred to as intron (intervening sequences)
b) The coding regions of RNA are referred to as exons
3. The process of removing introns and connecting exons is referred to as RNA
splicing
4. Prokaryotes do not contain introns
a) All eukaryotic genes examined thus far contain introns, with the exceptions
of RNA encoding histones and interferon
B. Mechanism
1. Type 1
a) Some RNA have been found to be autocatalytic
(1) This shocked most biologist, since it was generally believed that proteins were
the only biological catalysts
(2) RNA that can perform catalytic reaction are called ribozymes
b) This involves guanosine as a cofactor
c) So far observed in several protozoa
2. Type 2
a) Similar to type 1, but does not involve guanosine cofactors
b) Found in mitochondria and chloroplasts
3. Type 3
a) Used to splice nuclear RNA
b) Involves a structure referred to as spliceosomes
c) Characteristics of type 3 introns
(1) Introns often have a GU at its 5' end and AG at its 3' end
(a) This (and possibly other sequences) attract other spliceosome
components
d) Small nuclear RNAs (snRNAs)
(1) 100 - 200 nucleotides long
(2) Associate with nuclear proteins and are referred to as small nuclear
ribonucleoproteins (snRPPs, pronounced snurps)
(3) These are designated U1, U2, etc.
C. Function
1. Alternative splicing of primary RNA can result in different types of proteins
a) Examples
(1) Information for both the membrane-bound and secreted forms of antibodies
are both encoded on the same primary RNA transcript
(a) The final product will be determined on how they are spliced
(2) Information for both IgD and IgM classes of antibodies are both encoded on
the same primary RNA transcript
(a) The final product will be determined on how they are spliced
V.
RNA EDITING
A. Substitution editing
1. The bases in RNA are altered to different bases
2. Example
a) A 'C' in apolipoprotein in mammals is changed to a 'U'
b) An 'A' is changed to an 'I' (inosine -- which is read as a G during translation)
in mRNA encoding the mammalian brain glutamate receptor channel protein
B. Insertion / deletion editing
1. Bases are added or removed from RNA
2. Example
a) Trypanosomes add Us to many of its mitochondrial derived mRNAs
VI.TRANSPORT
TO THE CYTOPLASM
A. Messenger RNA, ribosomal RNA, and transfer RNAs leave the nucleus via the
nucleopores
1. It is estimated that about 5 rRNA and 1 mRNA molecule leaves each pore every
minute
2. Nucleopores are also where proteins enter the nucleus
a) All proteins are made in the cytoplasm of eukaryotes
(1) Histones, RNA and DNA polymerases and many regulatory proteins must
enter the nucleus