Ch11transcription

Ch11transcription - Transcrip)on
 •
RNA
synthesized
on
a
DNA
template,
catalyzed
by
 DNA‐dependent
RNA
polymerase


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Unformatted text preview: Transcrip)on
 •
RNA
synthesized
on
a
DNA
template,
catalyzed
by
 DNA‐dependent
RNA
polymerase
 •
ATP,
GTP,
CTP,
and
UTP
are
required,
as
is
Mg2+
 •
no
RNA
primer
is
required
 •
the
RNA
chain
is
synthesized
in
the
5’
‐>
3’
 direc)on;
the
nucleo)de
at
the
5’
end
of
the
chain
 retains
its
triphosphate
(ppp)
group
 •
the
DNA
base
sequence
contains
signals
for
 ini)a)on
and
termina)on
of
RNA
synthesis;
the
 enzyme
binds
to
and
moves
along
the
DNA
template
 in
the
3’
‐>
5’
direc)on
 •
the
DNA
template
is
unchanged
 Transcrip)on
in
prokaryotes
 The
Basics
of
Transcrip)on
 Promoter
sequence
 •  Simplest
of
organisms
contain
a
lot
of
DNA
that
is
 not
transcribed
 •  RNA
polymerase
needs
to
know
which
strand
is
 template
strand,
which
part
to
transcribe,
and
 where
first
nucleo)de
of
gene
to
be
transcribed
is
 •  Promoters‐DNA
sequence
that
provide
direc)on
 for
RNA
polymerase
 •  First
phase
of
 transcrip)on
is
 ini)a)on
 •  Ini)a)on
begins
when
 RNA
polymerase
binds
 to
promoter
and
forms
 closed
complex
 •  AVer
this,
DNA
 unwinds
at
promoter
 to
form
open
complex,
 which
is
required
for
 chain
ini)a)on
 •  AVer
strands
separated,
transcrip)on
bubble
of
~17
bp
moves
down
the
 DNA
sequence
to
be
transcribed
 •  RNA
polymerase
catalyzes
forma)on
of
phosphodiester
bonds
between
the
 incorp.
ribonucleo)des
 •  Topoisomerases
relax
supercoils
in
front
of
and
behind
transcrip)on
bubble
 •  Two
types
of
termina)on
mechanisms:
 
•
intrinsic
termina)on‐
controlled
by
specific
sequences,
 termina2on
sites
 •  Termina)on
sites
characterized
by
two
inverted
repeats
 •  Other
type
of
termina)on
involves
rho
(ρ)
protein
 •  Rho‐dependent
termina)on
sequences
cause
 hairpin
loop
to
form
 Transcrip)on
regula)on
in
prokaryotes
 •  In
prokaryotes,
transcrip)on
 regulated
by:

 

•

alterna)ve
σ
factors
 –  enhancers
 –  operons
 –  transcrip)on
a^enua)on
 •  Alterna2ve
σ
factors
 •  Viruses
and
bacteria
exert
 control
over
which
genes
are
 expressed
by
producing
 different
σ‐subunits
that
 direct
the
RNA
polymerase
to
 different
genes.
 Enhancers

 Certain
genes
include
sequences
upstream
of
extended
promoter
region
 These
genes
for
ribosomal
produc)on
have
3
upstream
sites,
Fis
sites
 Class
of
DNA
sequences
that
do
this
are
called
enhancers
 Bound
by
proteins
called
transcrip2on
factors
 Operon
 Basic
control
mechanisms
 Control
may
be
inducible
or
repressive,
and
 these
may
be
nega2vely
or
posi2vely
 controlled
 
Control
of
the
trp
operon
 Trp
operon
codes
for
a
leader
sequence
(trpL)
 and
five
polypep)des
 The
five
proteins
make
up
4
different
enzymes
 that
catalyze
the
mul)step
process
that
 converts
chorisimate
to
tryptophan
 Transcrip)on
in
eukaryotes
 Three
RNA
polymerases
are
known;
each
transcribes
a
different
 set
of
genes
and
recognizes
a
different
set
of
promoters:
 
•
RNA
Polymerase
I‐
found
in
the
nucleolus
and
synthesizes
 precursors
of
most
rRNAs
 
•
RNA
Polymerase
II‐
found
in
the
nucleoplasm
and
 synthesizes
mRNA
precursors
 
Most
studied
on
the
polymerases.
Consists
of
12
subunits. RPB‐
RNA
Polymerase
B
 
•
RNA
Polymerase
III‐
found
in
the
nucleoplasm
and
 synthesizes
tRNAs,
other
RNA
molecules
involved
in
mRNA
 processing
and
protein
transport
 How
does
pol
II
recognize
the
correct
 DNA

 Four
elements
of
the
Pol
II
promoter
allow
for
this
phenomenon
 Any
protein
regulator
of
transcrip)on
that
is
not
itself
a
subunit
of
Pol
II
is
a
 transcrip2on
factor
 Ini)a)on
begins
by
forming
the
preini2a2on
complex
 Transcrip)on
control
is
based
here
 Order
of
events
 Less
is
known
about
eukaryotes
 than
prokaryotes
 The
phosphorylated
Pol
II
 synthesizes
RNA
and
leaves
the
 promoter
region
behind
 GTFs
are
leV
at
the
promoter
or
 dissociate
from
Pol
II
 Elonga)on
and
termina)on
 •  Elonga2on
is
controlled
by:
 –  pause
sites,
where
RNA
Pol
will
hesitate
 –  an)‐termina)on,
which
proceeds
past
the
normal
 termina)on
point
 –  posi)ve
transcrip)on
elonga)on
factor
(P‐TEF)
 and
nega)ve
transcrip)on
elonga)on
factor
(N‐ TEF)
 •  Termina2on
 –  begins
by
stopping
RNA
Pol;
the
eukaryo)c
 consensus
sequence
for
termina)on
is
AAUAAA
 Gene
regula)on
 •  Enhancers
and
silencers‐
regulatory
 sequences
that
augment
or
diminish
 transcrip)on,
respec)vely
 •  DNA
looping
brings
enhancers
into
contact
 with
transcrip)on
factors
and
polymerase
 Eukaryo)c
gene
regula)on
 •  Response
elements
are
enhancers
that
respond
to
certain
metabolic
 factors
 
•
heat
shock
element
(HSE)
 
•
glucocor)coid
response
element
(GRE)
 
•
metal
response
element
(MRE)
 
•
cyclic‐AMP
response
element
(CRE)
 •  Response
elements
all
bind
proteins
(transcrip)on
factors)
that
are
 produced
under
certain
cell
condi)ons
 Structural
mo)fs
in
DNA
binding
 proteins
 •  Most
proteins
that
ac)vate
or
inhibit
RNA
Pol
 II
have
two
func)onal
domains:
 –  DNA‐binding
domain
 –  transcrip)on‐ac)va)on
domain
 •  DNA‐Binding
domains
have
domains
that
are
 either:
 
•
Helix‐Turn‐Helix
(HTH)
 
•
Zinc
fingers
 
•
Basic‐region
leucine
zipper
 Post
transcrip)onal
RNA
modifica)on
 •  tRNA,
rRNA,
and
mRNA
are
all
modified
aVer
transcrip)on
to
 give
the
func)onal
form
 –  the
ini)al
size
of
the
RNA
transcript
is
greater
than
the
 final
size
because
of
the
leader
sequences
at
the
5’
end
 and
the
trailer
sequences
at
the
3’
end
 –  the
types
of
processing
in
prokaryotes
can
differ
greatly
 from
that
in
eukaryotes,
especially
for
mRNA
 •  Modifica)ons
 –  trimming
of
leader
and
trailer
sequences
 –  addi)on
of
terminal
sequences
(a'er
transcrip.on)
 –  modifica)on
of
the
structure
of
specific
bases
(par.cularly
 in
tRNA)
 Ribozymes
 •  The
first
ribozymes
discovered
included
 those
that
catalyze
their
own
self‐splicing
 •  More
recently,
ribozymes
have
been
 discovered
that
are
involved
in
protein
 synthesis
 •  Group
I
ribozymes

 –  require
an
external
guanosine
 –  example:
pre‐rRNA
of
the
protozoan
 Tetrahymena
(next
screen)
 •  Group
II
ribozymes
 –  display
a
lariat
mechanism
similar
to
mRNA
 splicing
 –  no
requirement
for
an
external
nucleo)de
 ...
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This note was uploaded on 11/19/2010 for the course BIO 360 taught by Professor Fink during the Spring '10 term at UCLA.

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