Prokaryotes and eukaryotes alter gene expression in response to their changing environment. Natural selection has favored bacteria that produce only the products needed by that cell. Dependent on the types of nutrition they can absorb. If tryptophan levels stay high, the cell can stop synthesizing additional enzymes in this pathway by blocking transcription of the genes for these enzymes. High levels of tryptophan can inhibit the enzyme that is required to synthesized tryptophan. Operon: consists of: the genes that it controls: in bacteria, the genes coding for the enzymes of a particular pathway are often clustered together and transcribed as a single long mrna molecule. [polycistronic message: the promoter where rna polymerase binds, a single promoter regulates the synthesis of one polycistronic mrna, a(cid:374) operator (cid:449)hi(cid:272)h a(cid:272)ts as a(cid:374) (cid:862)o(cid:374)-off s(cid:449)it(cid:272)h(cid:863), lies (cid:271)et(cid:449)ee(cid:374) the pro(cid:373)oter a(cid:374)d the first gene. Operons allow coordinated regulation of related gene products.