Ask a Teacher

Search



what is lac operon and its functions?

The lac operon, an inducible operon, is a mechanism used by bacterial cells as an economical means to restrict the expression of the structural genes necessary for metabolizing lactose, a disaccharide. These structural genes break down lactose when lactose is the best carbon source available within its environs. E.coli utilizes the lac operon as a means of controlling the expression of its lac genes in response to its environment. The primary carbon source of this bacterium is glucose because it does not require a large amount of energy to metabolize. It is a more efficient source of energy than lactose. In the presence of both glucose and lactose, the bacterium will chose to metabolize glucose.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 ß-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. Lactose acts as an inducer for lac operon since it regulates the switching on and off of the operon.

If lactose is provided to the growth media of bacteria in absence of any other carbon source, then it is transported inside the cells by permease.

For permease to be present and lactose to enter inside the cells, low level of expression of lac operon must be present all the time.


comments powered by Disqus