Question

Which of the following is/are signal(s) for termination of transcription in prokaryotes?

I. The rho factor

II. Stem and loop RNA structures and a poly uracil sequence

III. The sigma factor

IV. Transcription factor TFIID

I and II only

I, II and III only

I, II, III, and IV

I, II, and IV only

Answer 1

Answer : I and II only

Explanation : Transcription termination in prokaryotes occurs in two different pathways : Rho-dependent termination and rho-independent.

In rho-dependent termination, the RNA transcript contains a site for rho factor. Rho factor binds to that site and start climbing up towards the RNA polymerase. When rho factor catches up the with the RNA polymerase at transcription bubble it releases the RNA transcript from DNA template and put an end to the transcription.

In rho-independent termination, rho factor is not involved. During transcription, the RNA polymerase hits a G-C rich region at the end of the coding region. When this G-C rich region is transcribed into RNA, the complementary G-C nucleotides bind together forming a hairpin like stem and loop structure. This stem and loop structure is followed by poly uracil sequence in the RNA which forms weak bond with the poly adenine sequence of DNA. The stem and loop structure causes the RNA polymerase to stall which results in enough instability to break the weak A=U bonds and release the RNA transcript from the DNA.

So, in prokaryotic transcription termination, there's no role of sigma factor and TFIID. Both of them helps in transcription initiation. TFIID is one of the several transcription factors that make up the RNA polymerase II preinitiation complex. Sigma factor helps in recognising the promoter region and binding of RNA polymerase to specific sequence.

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