Prokaryotes contain a circular genome where, in many cases, related genes for certain biochemical processes are located directly adjacent to each other in the DNA sequence. These gene clusters, known as operons, have a single promoter and are transcribed into a single mRNA. Expression of an operon is controlled by activators and repressors, proteins that promote or suppress transcription, respectively. An activator binds to a promoter, leading to the binding of RNA polymerase and the subsequent transcription of an operon. A repressor interacts with a different DNA sequence located in the vicinity of the promoter, known as the operator. When a repressor binds to the operator, it prevents RNA polymerase from binding to the promoter, inhibiting transcription. Prokaryotic gene regulation is often dependent on nutrient availability. In some bacteria when glucose concentrations are low, it leads to the accumulation of cyclic AMP. Cyclic AMP binds to an activator, the catabolite activator protein, and together they bind to a promoter. This initiates the expression of the lac operon, an operon containing genes that allow the bacteria to metabolize sugars other than glucose. Tryptophan is an amino acid required for protein synthesis. When it is not available from the environment, some organisms are capable of producing their own tryptophan. The genes for its synthesis are present in an operon that is regulated by a repressor. When tryptophan concentrations are high, it associates with the repressor. This complex can then bind to the operator, blocking RNA polymerase and suppressing transcription.