The lac operon is a set of genes expressed at the same time and under the regulation of the same regulatory mechanism. Operons occur more often in prokaryotes than eukaryotes because their genomes are smaller and simpler. They are an efficient way of saving resources as genes that are not needed are switched off.Three structural genes in E. coli - lacZ, lacY and lacA - code for β-galactosidase, lactose permease and transacetylase and are transcribed onto the same mRNA molecule. The gene lacI is near the operon and codes for a repressor protein. When the repressor protein is bound to the operator RNA polymerase cannot bind to the promoter. Lactose binds to the repressor protein and prevents it from binding to the operator.