This ppt tells you about the details of transcription in prokaryotes; the initiation, elongation and the termination steps. It not only covers the mRNA transcription, but also the rRNA and tRNA transcription.

1. Transcription in Prokaryotes

2. A transcription unit is a stretch of DNA that codes for an RNA molecule and
the sequence necessary for its transcription.
The transcription unit consists of three critical regions:
1. Promoter
2. RNA coding region
3. Terminator

3.  Bacterial RNA polymerase is a large
,multimeric enzyme consisting of
several polypeptide chains.
 Bacterial RNA polymerase has four
subunits that make up the core
enzyme: two copies of a subunit called
alpha, a single copy of beta and a
single copy of beta prime.
 Another functional subunit that joins
and leaves the core enzyme at the
particular process is the sigma factor.
 Association of sigma subunit to the core
enzyme forms the holoenzyme of the
RNA polymerase.

4. Transcription can be conveniently divided into three stages:-
INITIATION
ELONGATION
TERMINATION

7. INITIATION
1. RNA polymerase binds to DNA in a closed promoter complex.
2. The sigma factor allows the polymerase to convert the closed
promotercomplex to an open promoter complex.
3. The polymerase incorporates the first 9 to 10 nucleotides into the
nascent RNA.
4. The polymerase clears the promoter, loses its sigma factor and begins
the elongation phase.

8. EELONGATION
1. Elongation takes place at the transcription bubble,which moves along the
template strand.
2. The region containing RNA polymerase, DNA and nascent RNA is called
transcription bubble of DNA.
3. The polymerase enzyme elongates the growing RNA by adding the
rNMPs matching the code of the template.
4. The newly formed RNA forms a hybrid helix with the template DNA
strand.

9. TERMINATION
RNA synthesis will continue along the DNA template strand until the
polymerase encounters a signal that tells it to stop or terminate transcription.
RHO INDEPENDENT
RHO DEPENDENT

10. The rho-independent signal is found on the
DNA template strand and consists of a region
that contains a section that is then repeated a
few base pairs away in the inverted sequence.
As is shown in the figure, the patch is followed
by a short string of adenines. When this stretch
is transcribed into an RNA sequence, the RNA
can fold back and base pair with itself forming a
hairpin loop.
The string of adenines in the DNA sequence
are transcribed into uracils in the RNA
sequence. Because the uracil bases will only
pair weakly with the adenines, the RNA chain
can easily be released from the DNA template,
terminating transcription.

11. RHO DEPENDENT TERMINATION
• The rho-dependent terminator
received its name because it
is dependent on a specific
protein called a rho factor.
The rho factor is thought to
bind to the end of the RNA
chain and slide along the
strand towards the open
complex bubble. When the
factor catches the
polymerase, it causes the
termination of transcription.

12. rRNA Trancription

13. •n E. coli ribosomal RNA genes are organized
into seven operons, called rrn. These operons
have two promoters, P1 and P2, separated by
about 100bps.
•E.coli rRNA operons are transcribed by two
tandem promoters, rrn P1 and rrn P2, with P1
being the predominant promoter
•All seven P1 core promoters contain the
consensus -10 hexamer (TATAAT) and close
matches to the consensus -35 hexamer
(TTGACA)
•Each precursor RNAs contains non-coding
spacer and also tRNA segments are found in
spacers

14. The rrn P1 promoters consist of
three to five binding sites for the
transcription activator Fis, an UP
element (the binding site for the α
subunits of RNAP), and a core
promoter element (containing the
−10 and −35 hexamers for binding
the σ subunit of RNAP)

15. tRNA Transcription

16. • RNase-P recognizes the secondary structure, binds to it and cleaves exactly at 5’ end of the
tRNA
• At the other end RNase-D and endonucleases (first RNase E/Fs from 3’end) recognizes 3’
region and cuts, then RNaseD chops off nucleotide by nucleotide from 3’ end till it reaches
secondary structure with a --CCA 3’ sequence and stops
• The bases of tRNAs are then modified by a variety of enzymes. The modifications are
methylation, hydroxylation, thiolation, amination, deamination and few others.
• In some the tRNA liberated lacks CCA sequence at 3’ end. In such cases these nucleotides
are added in sequence by tRNA terminal transferase