Question

Please describe a torpedo model of RNA pol II transcription termination and explain why autocatalytic cleavage of the transcript is required

Answer 1

Solution: The termination of transcription occurs via two different methods. One is rho dependent(torpedo) and the second one is rho independend (allosteric). In the torpedo model, the RNA that is transcribed downstream from the Poly adenine site ( poly A site) is attacked by an exonuclease that degrades the DNA in 5prime - 3 prime direction. A DNA sequence that encodes a stretch of rna upstream of of the terminator that is devoid of any secondary structures. This unstructured part of rna serves as a binding site for the ancillary protein to bind and travel towards the region where polymerase synthesizing mRNA in a very low frequency. This ancillary protein destabilizes the rna-dna hybrid at transcription bubble. The second uncapped mRNA will be degraded soon after its emergence.