Question

Attenuation affects RNA polymerase through what mechanism? O 1. Accumulation of mutations 2.Concentration of a specific RNA 03. ATP levels in the cell 4. Rate of DNA replication

Answer 1

Attenuation is a process of premature termination of growing mRNA.

We will discuss with the example of Tryptophan Operon.

When stretch of DNA which is to be transcribed is recognized by the tryptophan repressor when the repressor binds to the operator it comes in the way of transcription enzyme which is usually RNA polymerase and blocks the operon from being transcribed when the Tryptophan level is high.

Repressor not always binds to the DNA but when Tryptophan is around it attaches to the repressor and some structural changes are made which activates the repressor and eventually blocks the transcription.

But when the Tryptophan level is low the Tryptophan repressor is inactive because there is no free tryptophan available to bind to it and block the transcription and hence transcription is processed to produce tryptophan.

Same way attenuation is a mechanism for reducing expression of tryptophan operon when there is more tryptophan present. But in this case instead of blocking the initiation of transcription, attenuation leads to premature termination of tryptophan operon usually prevents completion of transcription.

So, basically the attenuator sequence is present between the first gene of tryptophan operon and the operator known as Leader region. When the attenuator is transcribed into mRNA, it has self complementary regions and forms hair pin structures.

When RNA polymerase starts transcribing the operon ribosome attach to still forming transcript and begin translating the leader region. The polypeptide encoded by leader is 14 aminoacids long containing two tryptophan molecules. They are important because:

If there is high tryptophan the ribosome rapidly finishes the leader polypeptide transcription and does not wait for tRNA carrying the tryptophan. When ribosome finishes rapidly it will fall off after translation of leader polypeptide this allows formation of terminator hairpin loop causing the RNA polymerase to detach and end the transcription.

But, if there is low tryptophan the ribosome stalls at the tryptophan codons and will be slow in finishing he translation of leader polypeptide since it will be waiting for the tRNA carrying tryptophan. When there is a pause for tRNA carrying tryptophan there forms an anti termination hairpin loop which prevents formation of terminator loop and hence allowing the transcription to occur.

Concluding that by Rate of DNA replication mechanism attenuation affects the RNA polymerase, because the amount of DNA present provides the rate of mRNA to be produced. In this case the amount of mRNA produced is also based on amount of tryptophan present/absent which eventually results from DNA replication itself.