Question

During transcription, which of the following two mechanisms are used during the termination stage in bacteria?

a. Poly A tail; telomere formation

b. Release factor 1; release factor 2

c. Poly A tail; hairpin loop

d. hairpin loop; rho termination factor

Answer 1

Answer 2: “d” hairpin loop; rho termination factor

Transcription is formation of mRNA on DNA template. The termination of transcription in bacteria occurs in 2 ways, firstly by rho dependent factor and secondly by formation of stem loop structure.

Factor dependent termination involves a protein such as rho, that moves along the mRNA behind RNA polymerase and causes the RNA polymerase to dissociate from the DNA when it catches up to the open complex, where as factor independent termination involves formation of G-C rich stem loop structure that prevents the polymerase from continuing along the DNA.

Rho dependent termination:

Rho factor is required for factor-dependent transcription termination by RNA polymerase. Rho factor is hexametric protein. The transcription termination mechanism of Rho factor follows the below mentioned sequence.

Step 1: Rho factor binds to naked RNA at a C- rich site called as Rho utilization site also known as rut site using its primary binding region.

Step 2 The bound RNA reaches to secondary binding site and sensitizes Rho for ATP hydrolysis.

Step 3: The energy obtained from the ATP hydrolysis leads to activation of translocase activity of Rho factor and subsequently leads to unwinding of RNA/DNA duplex.

Rho independent termination ( hair pin loop structure)

In factor independent termination rho factor is not utilised, instead the enzyme catalysing the transcription i.e. RNA polymerase encounters a palindromic sequence on the DNA which is rich in G-C content, the RNA transcript formed on this part is self complementary and hence it forms a stem loop structure. The high G-C content favours the formation of stem and loop structure. This stem loop structure is followed by a sequence of 4 or more Uracil residues, which are critical for termination. The RNA transcript ends within or just after them.

Note: option b i.e. release factor 1 and release factor 2 are responsible for termination of translation(synthesis of polypeptide from mRNA)

option a i.e. poly A tail is the addition of adenine residues to the mRNA formed after transcription, this is seen in eukaryotes as a part of RNA processing after transcription.

Telomeres are the terminal ends of the chromosomes formed to protect the ends of the chromosomes by telomerase enzyme.