RNA (IE) and box C which recruits TFIIIA

RNA Polymerase III
Transcription:

RNA polymerase III (Pol III) transcribes non-coding RNA that
include tRNAs, 5S rRNA, U6 RNA, etc. The targets of Pol III were initially
believed to be housekeeping genes but recent discoveries have shown a role for Pol
III products in gene regulation 1.
The predominant Pol III transcripts are tRNAs and 5S rRNAs (accounting for…%)
which play essential roles in translation2.

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Structure:

RNA polymerase III is a massive 17 subunit and 700 KDa
protein complex (Table, yeast and
mammalian subunits). It’s the biggest of the RNA polymerases. All RNA
polymerases have five core subunits that are shared with archaeal and bacterial RNA polymerases. The three
eukaryotic polymerases share an additional five subunits to form the core ten-subunit
complex. They also share two subunits that form the stalk. In Pol I there are
two additional subunits that it shares with Pol III and thus has 14 subunits.
Pol III has an additional three subunits which makes it a 17 –subunit complex
(Table- pol I, II, III). One of the subunits of Pol III, Rpc31, share no homology with subunits of any of the
polymerases and is truly unique to Pol III.

 

Transcription
involves 3 steps: Initiation, elongation, and
termination

Initiation: Pol III uses gene internal promoters that
recruit transcription factors (Figure). There are three classes of promoters 3.

Type 1: Present in 5s rRNA gene contains an intermediate element (IE) and box C which
recruits TFIIIA which in turn recruits 6-subunit assembly factor TFIIIC and
then 3-subunit initiation factor TFIIIB leading to recruitment of Pol III to
the site.

Type 2: Identified in tRNAs, contain box A and box B
elements. The assembly factor TFIIIC binds to these elements which then allows
association of three subunit TFIIIB to these targets. The RNA polymerase is
then recruited to the site to form the pre-initiation complex 4.

Type 3: In higher eukaryotes, a significant number of Pol
III genes rely on gene external promoter elements including TATA box and PSE
(Proximal Sequence Element) that bind to an upstream binding factor called
SNAPc that functionally replaces TFIIIC5.
This further leads to the recruitment of the TFIIIB variant containing Brf2
instead of Brf16-8.

The Pol III subcomplex C31-C34-C82 is required for
transcription initiation as a C-terminal deletion of Rpc31 led to impaired
transcription in vivo while not
affecting elongation and termination 9.
It is also known that Rpc34 interacts directly with the TFIIIB subunit Brf1 and
mutations to Rpc34 are defective in Pol III recruitment and open complex
formation10.

Elongation:

Termination: The DNA sequence that signals termination
signal is a simple stretch of four or more Thymdines in yeast and six in
humans. The mechanism involves a oligo(dT)-induced conformational transition
mediated by Rpc53/37 subcomplex leading to increased pausing and facilitated
RNA release due to low stability of oligo 
(dA:rU) duplex 11.RNA Polymerase III
Transcription:

RNA polymerase III (Pol III) transcribes non-coding RNA that
include tRNAs, 5S rRNA, U6 RNA, etc. The targets of Pol III were initially
believed to be housekeeping genes but recent discoveries have shown a role for Pol
III products in gene regulation 1.
The predominant Pol III transcripts are tRNAs and 5S rRNAs (accounting for…%)
which play essential roles in translation2.

Structure:

RNA polymerase III is a massive 17 subunit and 700 KDa
protein complex (Table, yeast and
mammalian subunits). It’s the biggest of the RNA polymerases. All RNA
polymerases have five core subunits that are shared with archaeal and bacterial RNA polymerases. The three
eukaryotic polymerases share an additional five subunits to form the core ten-subunit
complex. They also share two subunits that form the stalk. In Pol I there are
two additional subunits that it shares with Pol III and thus has 14 subunits.
Pol III has an additional three subunits which makes it a 17 –subunit complex
(Table- pol I, II, III). One of the subunits of Pol III, Rpc31, share no homology with subunits of any of the
polymerases and is truly unique to Pol III.

 

Transcription
involves 3 steps: Initiation, elongation, and
termination

Initiation: Pol III uses gene internal promoters that
recruit transcription factors (Figure). There are three classes of promoters 3.

Type 1: Present in 5s rRNA gene contains an intermediate element (IE) and box C which
recruits TFIIIA which in turn recruits 6-subunit assembly factor TFIIIC and
then 3-subunit initiation factor TFIIIB leading to recruitment of Pol III to
the site.

Type 2: Identified in tRNAs, contain box A and box B
elements. The assembly factor TFIIIC binds to these elements which then allows
association of three subunit TFIIIB to these targets. The RNA polymerase is
then recruited to the site to form the pre-initiation complex 4.

Type 3: In higher eukaryotes, a significant number of Pol
III genes rely on gene external promoter elements including TATA box and PSE
(Proximal Sequence Element) that bind to an upstream binding factor called
SNAPc that functionally replaces TFIIIC5.
This further leads to the recruitment of the TFIIIB variant containing Brf2
instead of Brf16-8.

The Pol III subcomplex C31-C34-C82 is required for
transcription initiation as a C-terminal deletion of Rpc31 led to impaired
transcription in vivo while not
affecting elongation and termination 9.
It is also known that Rpc34 interacts directly with the TFIIIB subunit Brf1 and
mutations to Rpc34 are defective in Pol III recruitment and open complex
formation10.

Elongation:

Termination: The DNA sequence that signals termination
signal is a simple stretch of four or more Thymdines in yeast and six in
humans. The mechanism involves a oligo(dT)-induced conformational transition
mediated by Rpc53/37 subcomplex leading to increased pausing and facilitated
RNA release due to low stability of oligo 
(dA:rU) duplex 11.