Prokaryotic Transcription

Question 1

What is transcription?

Answer 1

Transcription is the process by which the information in a strand of DNA is copied into a new molecule of messenger RNA (mRNA)

https://microbenotes.com/prokaryotic-transcription-enzymes-steps-significance/

Question 2

What phases occur in prokaryotic transcription?

Answer 2

Initiation, Elongation, and Termination.

Question 3

What happens in the Initiation Phase?

Answer 3

1. The holoenzyme binds to a promoter region about 40-60 bp in size and then initiates transcription a short distance downstream (i.e 3 to the promoter)
2. Within the promoter lie two 6 base par sequences that are particularly important for promoter function. They are highly conserved between species.
3. The promoter elements lie at the positions of about 10 and 35 bp, respectively upstream of where transcription will begin.
4. The 10 bp sequence has the consensus because it was discovered by Pribnow, thus this element is known as the Pribnow box. This is an important recognition site that interacts with the σ factor of the RNA polymerase.
5. The 35 bp sequence has a consensus TTGACA and this is important in DNA unwinding during transcriptional initiation.
6. RNA polymerase does not need a primer to begin transcription; having a bound to the promoter site, the RNA polymerase begins transcription directly.

Question 4

What happens in the Elongation Phase?

Answer 4

1. After initiation, the σ factor is released from the transcriptional complex to leave the core enzymes (α2 ββω) which continues the elongation of the RNA transcript.
2. The first nucleotide in the RNA transcript is usually pppG or pppA.
3. The RNA polymerase then synthesizes RNA in the 5’ –> 3’ direction, using the for ribonucleotides 5-triphosphates (ATP, CTP, GTP, UTP) as precursors.
4. The 3-OH at the end of the growing RNA chain attacks the α phosphate group of the incoming ribonucleoside 5-triphosphate to form a 3’5′ phosphodiester bond.
5. The complex of RNA polymerase, DNA template and new RNA transcript is called a ternary complex (i.e. three components) and the region of unwound DNA that is undergoing transcription is called the transcription bubble.
6. The RNA transcript forms a transient RNA–DNA hybrid helix with its template strand but then peels away from the DNA as transcription proceeds.
7. The DNA is unwound ahead of the transcription bubble and after the transcription complex has passed, the DNA rewinds.
8. Thus, during the elongation, the RNA polymerase uses the antisense (-) strand of DNA as a template and synthesizes a complementary RNA molecule.
9. The RNA produced has the same sequence as the non-template strand, called the sense (+) strand (or coding strand) except that the RNA contains U instead of T.
10. At different locations on the bacterial chromosome, sometimes one strand is used as a template, sometimes the other, depending on which strand is the coding strand for the gene in question.
11. The correct strand to be used as a template is identified for the RNA polymerase by the presence of the promoter site.

Question 5

What are the core enzymes?

Answer 5

The core enzymes (α2 ββω) contain the catalytic site for polymerization, probably within the β sub-unit.

Question 6

What is the Termination phase?

Answer 6

Transcription continues until a termination sequence is reached.
1. The most common termination signal is a GC-rich region that is a palindrome, followed by an AT-rich sequence.
2. The RNA made from the DNA palindrome is self-complementary and so base pairs internally form a hairpin structure rich in GC base pairs followed by four or more U residues.
3. However, not all termination sites have this hairpin structure. Those that lack such a structure require an additional protein, called rho, to help recognize the termination site and stop transcription.
4. Thus the RNA polymerase encounters a termination signal and ceases transcription, releasing the RNA transcript and dissociating from the DNA

Question 7

RNA processing

Answer 7

• In prokaryotes, RNA transcribed from protein-coding genes (mRNA) requires little or no modification prior to translation.
• Many mRNA molecules begin to be translated even before RNA synthesis has finished.
• However, since ribosomal RNA (rRNA) and transfer RNA (tRNA) are synthesized as precursor molecules, they require post-transcriptional processing.