Friday, May 7, 2010

Bacterial Transcription: Control and Attenuation

A continuation from yesterday's post: Bacterial Transcription Initiation

Transcriptional Control and Attenuation
Like eukaryotes, bacteria must be able to control their gene activity.  Gene expression can be controlled at the transcriptional level in a few ways.  One is via alternative sigma factors (the protein that binds the -10 and -35 sites and positions RNA polymerase), which are involved in controlling expression of specialized operons.  For example, σ32 is involved in regulating heat shock genes, σ28 is for genes involved with motility and chemotaxis, σ54 is involved in nitrogen metabolism, and σ70 helps transcription of most genes. 

Another method of transcriptional control is via attenuation.  The most frequently cited example of attenuation is the trp operon, which has been studied extensively.  Initial observations indicated that when tryptophan was present for the bacteria, mRNA corresponding to the trp gene were short.  However, when tryptophan was limiting in the media, the mRNA transcript was longer.  If the researchers removed a short sequence of DNA, the mRNA was transcribed in full and genes were fully expressed.  This short sequence of DNA was termed the attenuator, or premature transcriptional stop. 

The trp operon codes for an mRNA with four different regions that can differentially bind to each other:  The second region can bind the first or third; the third can bind the second or fourth.  The first region contains two successive codons for tryptophan incorporation, which important for determining how the transcript is formed.  With high tryptophan, the ribosome moves along through the first region, without stopping at the successive tryptophan codons.  Because RNA polymerase has not had time to release the transcript before the ribosome translates through region one, causing regions three and four to bind, polymerase is forced off the mRNA and transcription is prematurely stopped.  This results in a shortened transcript when the cell has sufficient tryptophan.  In contrast, with low tryptophan levels in the cell, the ribosome will stall at the successive tryptophan codons because it is not able to quickly translate the mRNA.  This stalling allows for RNA polymerase to continue on its merry way and finish the full transcript because regions two and three (not three and four) bind.  

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