LAC AND TRP OPERON


Website:
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/L/LacOperon.html
[[http://www.ndsu.edu/instruct/mcclean/plsc431/prokaryo/prokaryo3.htm |http://www.ndsu.edu/instruct/mcclean/plsc431/prokaryo/prokaryo3.htm]]
http://mrsbracesbiowiki.wikispaces.com/Lac+Operon
http://pulsebiology02.wikispaces.com/lac+operon

The lac operon consists of a promoter, operator, and three structural genes which are Z, Y, and A. The lac operon is usually turned off and only with the help of the inducer is the lac operon on. This is why it is called an inducible operon. In order to turn the lac operon off the regulatory gene codes for the repressor protein which then bind to the operator. This ultimately prevents transcription from proceeding because the repressor protein on the operator prevents the RNA polymerase from moving forward from the promoter. In order for the lac operon to be on, the lactose needs to be converted into allolactose and then binds to the allosteric site of the repressor protein. This changes the shape of the repressor protein and thus prevents it from binding to the operator. Since there isn’t anything to prevent the RNA polymerase from transcribing the structural gene, the operon is on.

The trp operon consists of promoter, operator, and three structural genes which are E, D, C, B, and A. The trp operon is usually turned on and only with the help of the corepressor is the trp operon off. This is why it is called an repressible operon. In order to for the operon to be turn off, the regulatory gene codes for the repressor protein which shape doesn’t has the ability to bind to the operator. Without any obstacle to stop the RNA polymerase, it transcribes the structural gene. However, in order to turn the operon off, the tryptophan must act as a corepressor and bind itself to the allosteric site of the repressor protein. With the shape altered, the repressor is able to bind to the operator and thus prevents the RNA polymerase from transcribing the structural gene.

external image operon3.gif
http://www.boku.ac.at/iam/edu/gentech/operon3.gif
external image trp_operon.jpg
http://www.cofc.edu/poolel/trp_operon.jpg