Operons in Prokaryotes
Operons in prokaryotes consist of groups of genes that typically work together and are all controlled by one promoter, which is in turn controlled by an operator region. This means that all of the genes in the operon are turned on and off together.
As far as inducible operons go, the first example that comes to mind is the lac operon. The genes in the operon, lac Z, Y, and A, code for proteins that DEGRADE/utilize lactose. These genes are preceeded by an operator region. the operator region is the "control region" of the operon. With the lac operon, in the absence of lactose, an inhibitor (protein) binds to the operator region DNA, and the operon is effectively turned off because the inhibitor interferes with RNA polymerase binding. When lactose is around, it binds the inhibitor and releases it from the operator DNA. Once the repressing inhibitor is removed, RNA polymerase can bind and transcribe the operon (the Z,Y and A genes), and the operator is considered "induced" or turned on. If you look at this from the standpoint of the cell, it makes sense. The cell only needs the proteins that utilize lactose when lactose is around, right? So it’s set up that the genes that utilize lactose are only "turned on" when there is sufficient lactose in the cell (ie. the cell doesn’t waste energy making proteins it doesn’t need).
For repressible operons, the tryptophan operon comes to mind. In this operon, the genes for tryptophan (Trp) SYNTHESIS are grouped together. These genes are also preceeded by an operator region that controls expression of the operon. However, in this operon, there is a repressor that is only active (i.e., it will only bind the operator) when it is also bound to Trp. This means that there has to be a relatively high level of Trp in the cell to bind the repressor which will then turn off the genes for Trp production. This also makes sense from the standpoint of the cell….if you need more Trp, the genes will be turned on until there is enough Trp in the cell. When there is enough Trp in the cell, the genes will be turned off.
