Question

briefly describe one of the two major transcription termination methods, with special attention to the mechanism of termination. draw a diagram to aid.

Answer 1

Termination of transcription in prokaryotes

Sequences call Terminators Trigger the RNA polymerase to dissociate from DNA and release the RNA transcript it has synthesized. This whole action of Terminators is known as termination of transcription.

It occurs by 2 mechanisms

• Rho independent termination by intrinsic terminators and

• Rho dependent termination

1.Most rho-independent terminators have two distinguishing features.

• The first is a region that produces an RNA transcript with self-complementary sequences, permitting the formation of a hairpin structure centered 15 to 20 nucleotides before the projected end of the RNA strand.

• The second feature is a highly conserved string of three A residues in the template strand that are transcribed into U residues near the 3’ end of the hairpin. When a polymerase arrives at a termination site with this structure, it pauses. Formation of the hairpin structure in the RNA disrupts several AUU base pairs in the RNA-DNA hybrid segment and may disrupt important interactions between RNA and the RNA polymerase, facilitating dissociation of the transcript.

2. The rho-dependent terminators have two specificities

• lack the sequence of repeated A residues in the template strand but usually include a CA-rich sequence called a rut (rho utilization) element. These are 40 nucleotides long sequences that do not fold into secondary structure.

• The rho protein does not bind to a transcript being translated i.e., does not bind transcript bound to ribosomes. It typically terminates those transcript which are being translated beyond the end of genes.

The rho protein associates with the RNA at specific binding sites and migrates in the 5’ to 3’ direction until it reaches the transcription complex that is paused at a termination site. Here it contributes to release of the RNA transcript.

The rho protein has an ATP-dependent RNA-DNA helicase activity that promotes translocation of the protein along the RNA, and ATP is hydrolyzed by rho protein during the termination process.

I have given you details for both the types of mechanisms and image for only mechanism.

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