Figure 7–12 A cluster of bacterial genes can be transcribed from a single promoter. each of these five genes encodes a different enzyme, and all of these enzymes are needed to synthesize the amino acid tryptophan from simpler molecules. the genes are transcribed as a single mrnA molecule, a feature that allows their expression to be coordinated. Clusters of genes transcribed as a single mrnA molecule are common in bacteria. each of these clusters is called an operon because its expression is controlled by a cis-regulatory sequence called the operator (green), situated within the promoter. (in this and subsequent figures, the yellow blocks in the promoter represent DnA sequences that bind rnA polymerase; see figure 6–12).
start of transcription
We now understand exactly how this repression of the tryptophan operon comes about. Within the operon’s promoter is a cis-regulatory sequence that is recognized by a transcription regulator. When this regulator binds to this sequence, it blocks access of RNA polymerase to the promoter, thereby preventing transcription of the operon (and thus production of the tryptophan-producing enzymes). The transcription regulator is known as the tryptophan repressor and its cis-regulatory sequence is called the tryptophan operator. These components are controlled in a simple way: the repressor can bind to DNA only if it has also bound several molecules of tryptophan (Figure 7–13).
The tryptophan repressor is an allosteric protein, and the binding of tryptophan causes a subtle change in its three-dimensional structure so that the protein can bind to the operator sequence. Whenever the concentration of free tryptophan in the bacterium drops, tryptophan dissociates from the repressor, the repressor no longer binds to DNA, and the tryptophan operon is transcribed. The repressor is thus a simple device that switches production of a set of biosynthetic enzymes on and off according to the availability of the end product of the pathway that the enzymes catalyze.
The tryptophan repressor protein itself is always present in the cell. The gene that encodes it is continuously transcribed at a low level, so that a small amount of the repressor protein is always being made. Thus the bacterium can respond very rapidly to a rise or fall in tryptophan concentration.
The tryptophan repressor, as its name suggests, is a transcriptional repressor protein: in its active form, it switches genes off, or represses them. Some bacterial transcription regulators do the opposite: they switch genes on, or activate them. These transcriptional activator proteins work on promoters that—in contrast to the promoter for the tryptophan operon—are only marginally able to bind and position RNA polymerase on their own. However, these poorly functioning promoters can be made fully functional by activator proteins that bind to nearby cis-regulatory sequences and contact the RNA polymerase to help it initiate transcription (Figure 7–14).
Figure 7–13 Genes can be switched off by repressor proteins. if the concentration of tryptophan inside a bacterium is low (left), rnA polymerase (blue) binds to the promoter and transcribes the five genes of the tryptophan operon. however, if the concentration of tryptophan is high (right), the repressor protein (dark green) becomes active and binds to the operator (light green), where it blocks the binding of rnA polymerase to the promoter. whenever the concentration of intracellular tryptophan drops, the repressor falls off the DnA, allowing the polymerase to again transcribe the operon. Although not shown in the figure, the repressor is a stable dimer.