CONTROL OF GENE EXPRESSION [PROKAYOTES]

=Control of Gene Expression in Prokaryotes:= The following mechanism for controlling gene expression was the first to be discovered and is one of the best understood. It was discovered in a bacterium called Escherichia coli (E. coli for short) and concerns three genes coding for enzymes involved in the metabolism (break down) of Lactose (milk sugar). These genes and the sequences involved in regulating their expression form a region known as the Lac Operon. [image:http://i.imgur.com/dyzVVGF.png] RNA polymerase binds to the promoter, ready to transcribe the gene into mRNA. However, in the absence of lactose a repressor protein binds to short sequence just after the promoter known as the operator. The Operator literally blocks RNA polymerase preventing the transcription of the genes further down-stream. [image:http://i.imgur.com/Bf0aRYO.png] When lactose is present, it will diffuse into the bacterial cell and bind to the repressor. This induces a conformational (shape) change, releasing the repressor from the operator. [image:http://i.imgur.com/MIsjixh.png] As long as lactose is present, the repressor cannot bind and RNA polymerase is free to transcribe the gene (actually several RNA polymerases will transcribe the gene several times). The resulting mRNA is translated into the protein enzymes that break down (metabolise) lactose into glucose. [image:http://i.imgur.com/ROWpO9X.png] Once all the lactose has been metabolised, the repressor is once again able to bind to the operator, switching the system off again. ===Extra for Experts:=== If both glucose and lactose are available, it would make sense to use up all the glucose first, before expending valuable energy trying to break down the lactose. For this reason, E.coli have also developed another mechanism by which glucose (indirectly) prevents the RNA polymerase from binding, thus preventing transcription until all the glucose has been used.