Chapter 22
Nuclear splicing
22.1 Introduction
22.2 Nuclear splice junctions are short sequences
22.3 Splice junctions are read in pairs
22.4 Nuclear splicing proceeds through a lariat
22.5 snRNAs are required for splicing
22.6 U1 snRNP initiates splicing
22.7 The E complex can be formed in alternative ways
22.8 5 snRNPs form the spliceosome
22.9 An alternative splicing apparatus uses different snRNPs
22.10 Group II introns autosplice via lariat formation
22.11 Alternative splicing involves differential use of splice junctions
22.12 cis-splicing and trans-splicing reactions
22.13 Yeast tRNA splicing involves cutting and rejoining
22.14 The unfolded protein response is related to tRNA splicing
22.15 The 3￠ ends of polI and polIII transcripts are generated by termination
22.16 The 3￠ ends of mRNAs are generated by cleavage
22.17 Cleavage of the 3￠ end may require a small RNA
22.18 Production of rRNA requires cleavage and modification events
22.19 Small RNAs are required for rRNA processing
RNA splicing is the process of excising
the sequences in RNA that correspond
to introns,so that the sequences
corresponding to exons are connected
into a continuous mRNA.
pre-mRNA
heterogeneous nuclear RNA (hnRNA),
hnRNP
21.1 Introduction
Figure 22.1
hnRNA
exists as a
ribonucleopr
otein particle
organized as
a series of
beads.
22.1 Introduction
Figure 22.2 RNA is
modified in the nucleus by
additions to the 5￠ and 3￠
ends and by splicing to
remove the introns,The
splicing event requires
breakage of the exon-intron
junctions and joining of the
ends of the exons; the
expanded illustration shows
the principle schematically,
but not the actual order of
events,Mature mRNA is
transported through nuclear
pores to the cytoplasm,
where it is translated,
22.1 Introduction
GT-AG rule describes the presence of
these constant dinucleotides at the first
two and last two positions of introns of
nuclear genes.
Splice sites are the sequences
immediately surrounding the exon-intron
boundaries.
22.2 Nuclear splice junctions are
interchangeable but are read in pairs
Figure 22.3 The ends of nuclear
introns are defined by the GT-AG rule
22.2 Nuclear splice junctions are interchangeable but are
read in pairs
Figure 22.4
Splicing
junctions are
recognized
only in the
correct
pairwise
combinations,
22.2 Nuclear splice junctions are interchangeable but are
read in pairs
Figure 2.20 A special
splicing vector is used
for exon trapping,If an
exon is present in the
genomic fragment,its
sequence will be
recovered in the
cytoplasmic RNA,but
if the genomic
fragment consists
solely of an intron,
22.2 Nuclear splice junctions are interchangeable but are
read in pairs
Figure 22.5 Northern
blotting of nuclear RNA
with an ovomucoid probe
identifies discrete
precursors to mRNA,The
contents of the more
prominent bands are
indicated,Photograph
kindly provided by Bert
O'Malley,
22.2 Nuclear splice junctions
are interchangeable but are
read in pairs
Lariat is an intermediate in
RNA splicing in which a
circular structure with a tail
is created by a 5′-2′ bond.
22.3 Nuclear splicing proceeds
through a lariat
Figure 22.6
Splicing occurs in
two stages,in
which the 5￠
exon is separated
and then is joined
to the 3￠ exon,
22.3 Nuclear splicing
proceeds through a lariat
Figure 22.7
Nuclear splicing
occurs by two
transesterification
reactions in which
a free OH end
attacks a
phosphodiester
bond,
22.3 Nuclear splicing
proceeds through a
lariat
scRNA is any one of several small cytoplasmic
RNA molecules present in the cytoplasm and
(sometimes) nucleus.
snRNA (small nuclear RNA) is any one of
many small RNA species confined to the
nucleus; several of the snRNAs are involved in
splicing or other RNA processing reactions.
22.4 The spliceosome contains snRNAs
Figure 22.8 U1
snRNA has a base
paired structure
that creates several
domains,The 5￠
end remains single
stranded and can
base pair with the
5￠ splicing site,
22.4 The spliceosome
contains snRNAs
Figure 22.9
Mutations that
abolish function of
the 5￠ splicing site
can be suppressed by
compensating
mutations in U1
snRNA that restore
base pairing,
22.4 The spliceosome
contains snRNAs
Figure 22.10 The
splicing reaction
proceeds through
discrete stages in
which spliceosome
formation involves
the interaction of
components that
recognize the
consensus
sequences,
22.4 The
spliceosome
contains snRNAs
Figure 22.11 There may be multiple routes for initial
recognition of 5￠ and 3￠ splice sites.
22.4 The spliceosome contains snRNAs
Figure 22.12 U6-U4 pairing is
incompatible with U6-U2 pairing,
When U6 joins the spliceosome it
is paired with U4,Release of U4
allows a conformational change
in U6; one part of the released
sequence forms a hairpin (dark
grey),and the other part (black)
pairs with U2,Because an
adjacent region of U2 is already
paired with the branch site,this
brings U6 into juxtaposition with
the branch,Note that the
substrate RNA is reversed from
the usual orientation and is
shown 3￠ -5￠,
22.4 The
spliceosome
contains snRNAs
Figure 22.13
Splicing utilizes
a series of base
pairing reactions
between
snRNAs and
splice sites,
22.4 The spliceosome
contains snRNAs
Figure 22.17 Nuclear
splicing and group II
splicing involve the
formation of similar
secondary structures,
The sequences are more
specific in nuclear
splicing; group II
splicing uses positions
that may be occupied by
either purine (R) or
either pyrimidine (Y),
22.4 The spliceosome
contains snRNAs
Figure 22.14
Spliceosomes are
ellipsoidal particles
with several discrete
regions,The bar is
50 nm,Photograph
kindly provided by
Tom Maniatis,
22.4 The spliceosome
contains snRNAs
Figure 22.15 Three
classes of splicing
reactions proceed by two
transesterifications,First,
a free OH group attacks
the exon 1 - intron
junction,Second,the OH
created at the end of exon
1 attacks the intron - exon
2 junction.
22.5 Group II introns
autosplice via lariat
formation
Figure 22.6 Splicing
occurs in two stages,
in which the 5￠
exon is separated
and then is joined to
the 3￠ exon,
22.5 Group II introns
autosplice via lariat
formation
Figure 22.16
Splicing releases
mitochondrial group
II introns in the form
of stable lariats,
Photograph kindly
provided by Leslie
Grivell and Annika
Arnberg,
22.5 Group II introns
autosplice via lariat
formation
Figure 22.17 Nuclear
splicing and group II
splicing involve the
formation of similar
secondary structures,The
sequences are more specific
in nuclear splicing; group II
splicing uses positions that
may be occupied by either
purine (R) or either
pyrimidine (Y).
22.5 Group II introns
autosplice via lariat
formation
Figure 22.18 Alternative
forms of splicing may
generate a variety of
protein products from an
individual gene,
Changing the splice sites
may introduce
termination codons
(shown by asterisks) or
change reading frames,
22.6 Alternative splicing
involves differential use of
splice junctions
Figure 22.10 The
splicing reaction
proceeds through
discrete stages in
which spliceosome
formation involves
the interaction of
components that
recognize the
consensus
sequences,
22.6 Alternative
splicing involves
differential use of
splice junctions
Figure 22.19 Sex
determination in D,
melanogaster
involves a pathway
in which different
splicing events
occur in females,
Blocks at any stage
of the pathway
result in male
development,
22.6 Alternative
splicing involves
differential use of
splice junctions
Figure 22.20
Alternative
splicing events
that involve
both sites may
cause exons to
be added or
substituted,
22.6 Alternative
splicing involves
differential use of
splice junctions
Figure 22.21
Splicing usually
occurs only in
cis between
exons carried on
the same
physical RNA
molecule,but
trans splicing
can occur when
special
constructs are
made that
support base
pairing between
introns,
22.7 cis-splicing and trans-splicing reactions
Figure
22.11 There
may be
multiple
routes for
initial
recognition
of 5￠ and
3￠ splice
sites,
22.7 cis-splicing and trans-splicing reactions
Figure 22.22 The SL RNA
provides an exon that is
connected to the first exon
of an mRNA by trans-
splicing,The reaction
involves the same
interactions as nuclear cis-
splicing,but generates a
Y-shaped RNA instead of
a lariat.
22.7 cis-splicing
and trans-splicing
reactions
Figure 22.22 The SL RNA
provides an exon that is
connected to the first exon
of an mRNA by trans-
splicing,The reaction
involves the same
interactions as nuclear cis-
splicing,but generates a
Y-shaped RNA instead of
a lariat.
22.7 cis-splicing
and trans-splicing
reactions
Figure 22.23 The
intron in yeast
tRNAPhe base pairs
with the anticodon
to change the
structure of the
anticodon arm,
Pairing between an
excluded base in the
stem and the intron
loop in the
precursor may be
required for
splicing,
22.8 Yeast tRNA splicing involves cutting and rejoining
Figure 22.24
Splicing of
yeast tRNA in
vitro can be
followed by
assaying the
RNA precursor
and products by
gel
electrophoresis,
22.8 Yeast tRNA splicing involves cutting and rejoining
Figure 22.25 The 3￠
and 5￠ cleavages in S,
cerevisiae pre-tRNA
are catalyzed by
different subunits of
the endonuclease,
Another subunit may
determine location of
the cleavage sites by
measuring distance
from the mature
structure,The AI base
pair is also important,
22.8 Yeast tRNA splicing involves cutting and rejoining
Figure 22.26 Splicing of tRNA
requires separate nuclease and
ligase activities,The exon-
intron boundaries are cleaved
by the nuclease to generate
2￠ -3￠ cyclic phosphate and
5￠ OH termini,The cyclic
phosphate is opened to
generate 3￠ -OH and 2￠
phosphate groups,The 5￠ -
OH is phosphorylated,After
releasing the intron,the tRNA
half molecules fold into a
tRNA-like structure that now
has a 3￠ -OH,5￠ -P break,
This is sealed by a ligase,
22.8 Yeast tRNA splicing involves cutting and rejoining
Figure 22.27 The
unfolded protein
response occurs by
activating special
splicing of HAC1
mRNA to produce
a transcription
factor that
recognizes the
UPR.
22.8 Yeast tRNA
splicing involves cutting
and rejoining
Figure 22.28 When a
3￠ end is generated
by termination,RNA
polymerase and
RNA are released at
a discrete (terminator)
sequence in DNA,
22.9 The 3 ends of polI
and polIII transcripts
are generated by
termination
Figure 22.29 When a
3￠ end is generated
by cleavage,RNA
polymerase
continues
transcription while
an endonuclease
cleaves at a defined
sequence in the RNA,
22.9 The 3 ends of polI
and polIII transcripts
are generated by
termination
Cordycepin is 3′ deoxyadenosine,an
inhibitor of polyadenylation of RNA.
Endonucleases cleave bonds within a
nucleic acid chain; they may be specific
for RNA or for single-stranded or
double-stranded DNA.
22.10 The 3 ends of mRNAs are
generated by cleavage
Figure 22.30 The
sequence
AAUAAA is
necessary for
cleavage to
generate a 3￠ end
for
polyadenylation,
22.10 The 3 ends of
mRNAs are generated
by cleavage
Figure 22.31 The 3￠
processing complex
consists of several
activities,CPSF and
CstF each consist of
several subunits; the
other components are
monomeric,The total
mass is >900 kD,
22.10 The 3 ends of
mRNAs are generated
by cleavage
Figure 22.32
Generation of
the 3￠ end of
histone H3
mRNA
depends on a
conserved
hairpin and a
sequence that
base pairs with
U7 snRNA,
22.11 Cleavage of the 3 end may require a small RNA
Figure 22.33
Mature rRNAs
are generated
by cleavage
and trimming
events from a
primary
transcript
22.12 Production of
rRNA requires cleavage
and modification events
Figure 22.34 The rrn operons 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.
22.12 Production of rRNA requires cleavage and
modification events
Figure 22.35 A
snoRNA base
pairs with a
region of
rRNA that is to
be methylated.
22.13 Small RNAs
are required for
rRNA processing
Figure 22.36 An ACA group snoRNA base pairs with rRNA
to determine the position of pseudouridine modification,
22.13 Small RNAs are required for rRNA processing
1,Splicing accomplishes the removal of introns and the
joining of exons into the mature sequence of RNA.
2,Nuclear splicing follows preferred but not obligatory
pathways.
3,Nuclear splicing requires formation of a spliceosome,a
large particle that assembles the consensus sequences into
a reactive conformation,
4,Splicing is usually intramolecular,but some cases have
been found of trans- (intermolecular) splicing.
Summary
5,Group II introns share with nuclear introns the use
of a lariat as intermediate,but are able to perform the
reaction as a self-catalyzed property of the RNA,
6,Yeast tRNA splicing involves separate
endonuclease and ligase reactions,
7,The termination capacity of RNA polymerase II
has not been characterized,and 3 ends of its
transcripts are generated by cleavage.
Summary
Nuclear splicing
22.1 Introduction
22.2 Nuclear splice junctions are short sequences
22.3 Splice junctions are read in pairs
22.4 Nuclear splicing proceeds through a lariat
22.5 snRNAs are required for splicing
22.6 U1 snRNP initiates splicing
22.7 The E complex can be formed in alternative ways
22.8 5 snRNPs form the spliceosome
22.9 An alternative splicing apparatus uses different snRNPs
22.10 Group II introns autosplice via lariat formation
22.11 Alternative splicing involves differential use of splice junctions
22.12 cis-splicing and trans-splicing reactions
22.13 Yeast tRNA splicing involves cutting and rejoining
22.14 The unfolded protein response is related to tRNA splicing
22.15 The 3￠ ends of polI and polIII transcripts are generated by termination
22.16 The 3￠ ends of mRNAs are generated by cleavage
22.17 Cleavage of the 3￠ end may require a small RNA
22.18 Production of rRNA requires cleavage and modification events
22.19 Small RNAs are required for rRNA processing
RNA splicing is the process of excising
the sequences in RNA that correspond
to introns,so that the sequences
corresponding to exons are connected
into a continuous mRNA.
pre-mRNA
heterogeneous nuclear RNA (hnRNA),
hnRNP
21.1 Introduction
Figure 22.1
hnRNA
exists as a
ribonucleopr
otein particle
organized as
a series of
beads.
22.1 Introduction
Figure 22.2 RNA is
modified in the nucleus by
additions to the 5￠ and 3￠
ends and by splicing to
remove the introns,The
splicing event requires
breakage of the exon-intron
junctions and joining of the
ends of the exons; the
expanded illustration shows
the principle schematically,
but not the actual order of
events,Mature mRNA is
transported through nuclear
pores to the cytoplasm,
where it is translated,
22.1 Introduction
GT-AG rule describes the presence of
these constant dinucleotides at the first
two and last two positions of introns of
nuclear genes.
Splice sites are the sequences
immediately surrounding the exon-intron
boundaries.
22.2 Nuclear splice junctions are
interchangeable but are read in pairs
Figure 22.3 The ends of nuclear
introns are defined by the GT-AG rule
22.2 Nuclear splice junctions are interchangeable but are
read in pairs
Figure 22.4
Splicing
junctions are
recognized
only in the
correct
pairwise
combinations,
22.2 Nuclear splice junctions are interchangeable but are
read in pairs
Figure 2.20 A special
splicing vector is used
for exon trapping,If an
exon is present in the
genomic fragment,its
sequence will be
recovered in the
cytoplasmic RNA,but
if the genomic
fragment consists
solely of an intron,
22.2 Nuclear splice junctions are interchangeable but are
read in pairs
Figure 22.5 Northern
blotting of nuclear RNA
with an ovomucoid probe
identifies discrete
precursors to mRNA,The
contents of the more
prominent bands are
indicated,Photograph
kindly provided by Bert
O'Malley,
22.2 Nuclear splice junctions
are interchangeable but are
read in pairs
Lariat is an intermediate in
RNA splicing in which a
circular structure with a tail
is created by a 5′-2′ bond.
22.3 Nuclear splicing proceeds
through a lariat
Figure 22.6
Splicing occurs in
two stages,in
which the 5￠
exon is separated
and then is joined
to the 3￠ exon,
22.3 Nuclear splicing
proceeds through a lariat
Figure 22.7
Nuclear splicing
occurs by two
transesterification
reactions in which
a free OH end
attacks a
phosphodiester
bond,
22.3 Nuclear splicing
proceeds through a
lariat
scRNA is any one of several small cytoplasmic
RNA molecules present in the cytoplasm and
(sometimes) nucleus.
snRNA (small nuclear RNA) is any one of
many small RNA species confined to the
nucleus; several of the snRNAs are involved in
splicing or other RNA processing reactions.
22.4 The spliceosome contains snRNAs
Figure 22.8 U1
snRNA has a base
paired structure
that creates several
domains,The 5￠
end remains single
stranded and can
base pair with the
5￠ splicing site,
22.4 The spliceosome
contains snRNAs
Figure 22.9
Mutations that
abolish function of
the 5￠ splicing site
can be suppressed by
compensating
mutations in U1
snRNA that restore
base pairing,
22.4 The spliceosome
contains snRNAs
Figure 22.10 The
splicing reaction
proceeds through
discrete stages in
which spliceosome
formation involves
the interaction of
components that
recognize the
consensus
sequences,
22.4 The
spliceosome
contains snRNAs
Figure 22.11 There may be multiple routes for initial
recognition of 5￠ and 3￠ splice sites.
22.4 The spliceosome contains snRNAs
Figure 22.12 U6-U4 pairing is
incompatible with U6-U2 pairing,
When U6 joins the spliceosome it
is paired with U4,Release of U4
allows a conformational change
in U6; one part of the released
sequence forms a hairpin (dark
grey),and the other part (black)
pairs with U2,Because an
adjacent region of U2 is already
paired with the branch site,this
brings U6 into juxtaposition with
the branch,Note that the
substrate RNA is reversed from
the usual orientation and is
shown 3￠ -5￠,
22.4 The
spliceosome
contains snRNAs
Figure 22.13
Splicing utilizes
a series of base
pairing reactions
between
snRNAs and
splice sites,
22.4 The spliceosome
contains snRNAs
Figure 22.17 Nuclear
splicing and group II
splicing involve the
formation of similar
secondary structures,
The sequences are more
specific in nuclear
splicing; group II
splicing uses positions
that may be occupied by
either purine (R) or
either pyrimidine (Y),
22.4 The spliceosome
contains snRNAs
Figure 22.14
Spliceosomes are
ellipsoidal particles
with several discrete
regions,The bar is
50 nm,Photograph
kindly provided by
Tom Maniatis,
22.4 The spliceosome
contains snRNAs
Figure 22.15 Three
classes of splicing
reactions proceed by two
transesterifications,First,
a free OH group attacks
the exon 1 - intron
junction,Second,the OH
created at the end of exon
1 attacks the intron - exon
2 junction.
22.5 Group II introns
autosplice via lariat
formation
Figure 22.6 Splicing
occurs in two stages,
in which the 5￠
exon is separated
and then is joined to
the 3￠ exon,
22.5 Group II introns
autosplice via lariat
formation
Figure 22.16
Splicing releases
mitochondrial group
II introns in the form
of stable lariats,
Photograph kindly
provided by Leslie
Grivell and Annika
Arnberg,
22.5 Group II introns
autosplice via lariat
formation
Figure 22.17 Nuclear
splicing and group II
splicing involve the
formation of similar
secondary structures,The
sequences are more specific
in nuclear splicing; group II
splicing uses positions that
may be occupied by either
purine (R) or either
pyrimidine (Y).
22.5 Group II introns
autosplice via lariat
formation
Figure 22.18 Alternative
forms of splicing may
generate a variety of
protein products from an
individual gene,
Changing the splice sites
may introduce
termination codons
(shown by asterisks) or
change reading frames,
22.6 Alternative splicing
involves differential use of
splice junctions
Figure 22.10 The
splicing reaction
proceeds through
discrete stages in
which spliceosome
formation involves
the interaction of
components that
recognize the
consensus
sequences,
22.6 Alternative
splicing involves
differential use of
splice junctions
Figure 22.19 Sex
determination in D,
melanogaster
involves a pathway
in which different
splicing events
occur in females,
Blocks at any stage
of the pathway
result in male
development,
22.6 Alternative
splicing involves
differential use of
splice junctions
Figure 22.20
Alternative
splicing events
that involve
both sites may
cause exons to
be added or
substituted,
22.6 Alternative
splicing involves
differential use of
splice junctions
Figure 22.21
Splicing usually
occurs only in
cis between
exons carried on
the same
physical RNA
molecule,but
trans splicing
can occur when
special
constructs are
made that
support base
pairing between
introns,
22.7 cis-splicing and trans-splicing reactions
Figure
22.11 There
may be
multiple
routes for
initial
recognition
of 5￠ and
3￠ splice
sites,
22.7 cis-splicing and trans-splicing reactions
Figure 22.22 The SL RNA
provides an exon that is
connected to the first exon
of an mRNA by trans-
splicing,The reaction
involves the same
interactions as nuclear cis-
splicing,but generates a
Y-shaped RNA instead of
a lariat.
22.7 cis-splicing
and trans-splicing
reactions
Figure 22.22 The SL RNA
provides an exon that is
connected to the first exon
of an mRNA by trans-
splicing,The reaction
involves the same
interactions as nuclear cis-
splicing,but generates a
Y-shaped RNA instead of
a lariat.
22.7 cis-splicing
and trans-splicing
reactions
Figure 22.23 The
intron in yeast
tRNAPhe base pairs
with the anticodon
to change the
structure of the
anticodon arm,
Pairing between an
excluded base in the
stem and the intron
loop in the
precursor may be
required for
splicing,
22.8 Yeast tRNA splicing involves cutting and rejoining
Figure 22.24
Splicing of
yeast tRNA in
vitro can be
followed by
assaying the
RNA precursor
and products by
gel
electrophoresis,
22.8 Yeast tRNA splicing involves cutting and rejoining
Figure 22.25 The 3￠
and 5￠ cleavages in S,
cerevisiae pre-tRNA
are catalyzed by
different subunits of
the endonuclease,
Another subunit may
determine location of
the cleavage sites by
measuring distance
from the mature
structure,The AI base
pair is also important,
22.8 Yeast tRNA splicing involves cutting and rejoining
Figure 22.26 Splicing of tRNA
requires separate nuclease and
ligase activities,The exon-
intron boundaries are cleaved
by the nuclease to generate
2￠ -3￠ cyclic phosphate and
5￠ OH termini,The cyclic
phosphate is opened to
generate 3￠ -OH and 2￠
phosphate groups,The 5￠ -
OH is phosphorylated,After
releasing the intron,the tRNA
half molecules fold into a
tRNA-like structure that now
has a 3￠ -OH,5￠ -P break,
This is sealed by a ligase,
22.8 Yeast tRNA splicing involves cutting and rejoining
Figure 22.27 The
unfolded protein
response occurs by
activating special
splicing of HAC1
mRNA to produce
a transcription
factor that
recognizes the
UPR.
22.8 Yeast tRNA
splicing involves cutting
and rejoining
Figure 22.28 When a
3￠ end is generated
by termination,RNA
polymerase and
RNA are released at
a discrete (terminator)
sequence in DNA,
22.9 The 3 ends of polI
and polIII transcripts
are generated by
termination
Figure 22.29 When a
3￠ end is generated
by cleavage,RNA
polymerase
continues
transcription while
an endonuclease
cleaves at a defined
sequence in the RNA,
22.9 The 3 ends of polI
and polIII transcripts
are generated by
termination
Cordycepin is 3′ deoxyadenosine,an
inhibitor of polyadenylation of RNA.
Endonucleases cleave bonds within a
nucleic acid chain; they may be specific
for RNA or for single-stranded or
double-stranded DNA.
22.10 The 3 ends of mRNAs are
generated by cleavage
Figure 22.30 The
sequence
AAUAAA is
necessary for
cleavage to
generate a 3￠ end
for
polyadenylation,
22.10 The 3 ends of
mRNAs are generated
by cleavage
Figure 22.31 The 3￠
processing complex
consists of several
activities,CPSF and
CstF each consist of
several subunits; the
other components are
monomeric,The total
mass is >900 kD,
22.10 The 3 ends of
mRNAs are generated
by cleavage
Figure 22.32
Generation of
the 3￠ end of
histone H3
mRNA
depends on a
conserved
hairpin and a
sequence that
base pairs with
U7 snRNA,
22.11 Cleavage of the 3 end may require a small RNA
Figure 22.33
Mature rRNAs
are generated
by cleavage
and trimming
events from a
primary
transcript
22.12 Production of
rRNA requires cleavage
and modification events
Figure 22.34 The rrn operons 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.
22.12 Production of rRNA requires cleavage and
modification events
Figure 22.35 A
snoRNA base
pairs with a
region of
rRNA that is to
be methylated.
22.13 Small RNAs
are required for
rRNA processing
Figure 22.36 An ACA group snoRNA base pairs with rRNA
to determine the position of pseudouridine modification,
22.13 Small RNAs are required for rRNA processing
1,Splicing accomplishes the removal of introns and the
joining of exons into the mature sequence of RNA.
2,Nuclear splicing follows preferred but not obligatory
pathways.
3,Nuclear splicing requires formation of a spliceosome,a
large particle that assembles the consensus sequences into
a reactive conformation,
4,Splicing is usually intramolecular,but some cases have
been found of trans- (intermolecular) splicing.
Summary
5,Group II introns share with nuclear introns the use
of a lariat as intermediate,but are able to perform the
reaction as a self-catalyzed property of the RNA,
6,Yeast tRNA splicing involves separate
endonuclease and ligase reactions,
7,The termination capacity of RNA polymerase II
has not been characterized,and 3 ends of its
transcripts are generated by cleavage.
Summary
