The lac operon

(a) This diagram depicts a region of the E. coli chromosome that contains the lacI regulatory gene and the adjacent lac operon, as well as the polycistronic mRNA transcribed from the operon. (b) Function of lactose permease and ?-galactosidase. Lactose permease cotransports H+ with lactose. Bacteria maintain an H+ gradient across their cytoplasmic membrane that drives the active transport of lactose into the cytoplasm. ?-galactosidase cleaves lactose into galactose and glucose. As a side reaction, it can also convert lactose into allolactose.