Essay 7- The Lac Operon
Year 2 Biomedical Science
“Discuss how the Escherichia coli lac operon is controlled.”
It is well known that bacterial do not express all their genes all the time to save resources. However the bacteria need to respond quickly to changes in the environment, these changes can be met by grouped genes which encode proteins that work together known as operons. E.coli is one of the most studied organisms worldwide, especially the study of its operons. E.coli can digest the sugar lactose, the genes which it needs to do this are inducible by the inducer, lactose. E.coli only needs the proteins to digest lactose when lactose is present. The lac operon consists of a few key genes for proteins, this proteins include: B-galactosidase, lactose permease and A-tranacetylase, which are translated from LacZ, LacY and LacA genes respectively.
B-galatosidase is an enzyme which cleaves lactose into galactose and glucose, it also catalyses the isomerisation of lactose to allolactose, a compound important in regulating the expression of the lac operon. Lactose permease Is a membrane bound proteins that brings lactose into the cell, and A-tranacetlyase rids the cell of toxic thiogalatosides that are taken up by the permease. In E.coli in a medium containing glucose, only a low concentration of each of these three proteins is produces as glucose is favoured over lactose. However in the presence of lactose and absence of glucose there are about 3,000 molecules of b-galactosidase per cell. The genes mentioned above, are transcribed into mRNA’s: these mRNA’s have short half-lives so the transcripts for these must be made continually in order for the enzymes to be produced. When lactose is no longer present production of mRNA is stopped and the mRNA already present is broken down so no more protein is made. On the lac operon there is also a lacl- gene present, this is the regulatory gene which is located close to the structural gene. Lacl- codes for a repressor molecule which controls the negative control of the lac operon system. The promoter for lacl- is weak so only a few repressor molecules are made, however these repressor molecules are made constitutively and bind to the operator. Once bound to the operator the lac repressor prevents transcription of mRNA as RNA polymerase cannot bind to the promoter when the repressor is bound to the operator (the operator modulates the expression of the...