An example is the "trp" gene in bacteria. When there is a high level of tryptophan in the region, it is inefficient for the bacterium to synthesize more. When the RNA polymerase binds and transcribes the "trp" gene, the ribosome will start translating. (This differs from eukaryotic cells, where RNA must exit the nucleus before translation starts.) The attenuator sequence, which is located between the mRNA leader sequence (5' UTR) and trp operon gene sequence, contains four domains, where domain 3 can pair with domain 2 or domain 4.