The lac operon consists of three structural genes, Z, Y and A, which code for the enzymes necessary to catabolize lactose and a regulator gene, I, which codes for a regulator protein. The regulator protein regulates the operon. It turns the operon “on” or “off” depending upon the presence of lactose by binding to an operator region. The structural gene, Z, codes for ฿-galactosidase which is primarily responsible for the hydrolysis of lactose into its monomeric units, galactose and glucose. The Y gene codes for the enzyme lac permease which functions specifically in the transport of lactose into the bacterial cell. The cell is otherwise impermeable to lactose. The A gene codes for transacetylase, whose function is controlled coordinately with the Z and Y genes. The structural genes are under the control of a promoter, P, onto which RNA polymerase binds. It is necessary for the transcription of the structural genes. The RNA polymerase binds to the promoter and transcribes a single continuous mRNA molecule for all three structural genes. Because the mRNA molecule contains the transcripts of more than one gene, it is called a polycistronic or polygenic mRNA.