We will start with transcription termination as our first downstream mechanism, and we will use an example from bacteria, where this process is best understood. The most frequently used mechanism is called Rho-independent termination, and this is where we will concentrate. There is another mechanism, Rho-dependent termination, which uses a protein called Rho as a transcription termination factor. Note that both of these characteristic features are encoded in the termination site DNA, but we recognize their consequences in the RNA transcript. The stem-loop structure that is formed by the GC base pairs n the RNA transcript is thought to bind to the E. coli RNA polymerase and cause it to pause during transcription.
Attenuation provides a secondary mechanism for controlling expression of the trp operon.
Recall that this operon encodes the genetic information to make the biosynthetic enzymes that produce the amino acid tryptophan in bacterial cells. In cells with high trp levels, a short piece of RNA is transcribed, i.e. transcription is attenuated. In cells with low trp levels, the complete mRNA is made. Mechanism of attenuation of trp-operon transcription. To understand the attenuation mechanism, recall that in bacteria, translation can start at the 5' end of mRNA even before it is finished being transcribed.