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Introduction
to Molecular and Cell Biology, Biol. 220
Lecture 21: Transcription Termination
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In Prokaryotes
The Rho-Independent termination signals
- The intrinsic terminator sequence is an inverted
repeat of GC-rich sequence followed by 4 or more adenines. The transcribed
RNA forms stem-loop structure at inverted repeats via internal
base pairing
- The formation of this stem-loop structure (Fig.
11.1) disrupts hydrogen bonding between RNA uracils and DNA adenines at
site of transcription (weak because only 2 H-bonds between A and U as compared
to 3 between G and C)
- As a result, RNA is released from the DNA template
- Fig. 11.1
What is an inverted repeat? A sequence of several bases in double-stranded
DNA that is repeated in an inverted fashion
Example:
- 5'.....GCCGCCAG........CTGGCGGC....3'
- 3'.....CGGCGGTC........GACCGCCG....5' (template
strand)
transcribed RNA: 5'.......GCCGCCAG........CTGGCGGC.....3'
Consequently there are internal sequences in the
transcribed RNA that are complementary and can therefore base pair to form a
stem-loop structure (Fig. 11.1).
Rho-dependent Termination Signals
- Some termination sequences lack the series of adenines
which are transcribed in to URACILS on the RNA. The RNA in such situations
needs assistance from a specific protein (termedRho) which is necessary
for termination.
- Rho binds at the 5'end of the RNA and scans down the
RNA until it catches up with an RNA polymerase which is paused at a
stem-loop structure.
- In Rho dependent termination, the Rho protein forces
the RNA to separate from the DNA template.
Eukaryotic Transcriptional Termination
- RNA polymerase I terminates when it comes to a polymerase-specific
DNA binding protein attached at the termination site.
- RNA polymerase III terminates at a series of U residues
but does not require an upstream stem-loop be present in the mRNA.
- RNA polymerase II transcripts are essentially terminated
by the cleavage near the polyadenylation site followed by the addition of
the poly(A) tail (Fig. 11.12).
The cleaved 3' transcriptional product is rapidly degraded as are un-polyadenylated
transcripts.
Fig. 11.12
Transcription Regulation that involves transcriptional
termination
Attenuation
- Attenuation provides a secondary mechanism for controlling
expression of the prokaryotic trp operon (Fig.
11.2).
Fig. 11.2
- Attenuation requires simultaneous transcription and
translation (Fig. 11.3) and therefore
only occurs in prokaryotes.
- Fig. 11.3
In the presence of trp-charged tRNA leader sequence is
closely translated behind RNA polymerase (Fig.
11.3, left figure).
When trp-charged tRNA is low the ribosome pauses before
end of leader and allows alternative stem-loop to form that prevents termination
(Fig. 11.3, right figure).
Antitermination
- In Prokaryotes, binding of antitermination proteins,
such as the N protein of lambda, between the promoter and the terminator allows
a protein complex to form with nus proteins and prevent termination.
- In Prokaryotes, ntiterminators work at Rho-dependent
and Rho-independent terminators.
- In eukaryotic cells transcribing he HIV genome the
Tat protein functions as an antiterminator (Fig.
11.6) by binding at the TAR site near the 5' end of the transcript.
Without Tat, only short RNAs from the 5' end of the genome are transcribed.
Fig. 11.6
