1、THE lac OPERON, 2007 Paul Billiet ODWS,The control of gene expression,Each cell in the human contains all the genetic material for the growth and development of a human Some of these genes will be need to be expressed all the time These are the genes that are involved in of vital biochemical process
2、es such as respiration Other genes are not expressed all the time They are switched on an off at need, 2007 Paul Billiet ODWS,Operons,An operon is a group of genes that are transcribed at the same time. They usually control an important biochemical process. They are only found in prokaryotes., Nobel
3、Prize.org,Jacob, Monod & Lwoff, 2007 Paul Billiet ODWS,The lac Operon,The lac operon consists of three genes each involved in processing the sugar lactose One of them is the gene for the enzyme -galactosidase This enzyme hydrolyses lactose into glucose and galactose, 2007 Paul Billiet ODWS,Adapting
4、to the environment,E. coli can use either glucose, which is a monosaccharide, or lactose, which is a disaccharide However, lactose needs to be hydrolysed (digested) first So the bacterium prefers to use glucose when it can, 2007 Paul Billiet ODWS,Four situations are possible,When glucose is present
5、and lactose is absent the E. coli does not produce -galactosidase. When glucose is present and lactose is present the E. coli does not produce -galactosidase. When glucose is absent and lactose is absent the E. coli does not produce -galactosidase. When glucose is absent and lactose is present the E
6、. coli does produce -galactosidase, 2007 Paul Billiet ODWS,The control of the lac operon, 2007 Paul Billiet ODWS,1. When lactose is absent,A repressor protein is continuously synthesised. It sits on a sequence of DNA just in front of the lac operon, the Operator site The repressor protein blocks the
7、 Promoter site where the RNA polymerase settles before it starts transcribing, 2007 Paul Billiet ODWS,2. When lactose is present,A small amount of a sugar allolactose is formed within the bacterial cell. This fits onto the repressor protein at another active site (allosteric site) This causes the re
8、pressor protein to change its shape (a conformational change). It can no longer sit on the operator site. RNA polymerase can now reach its promoter site, 2007 Paul Billiet ODWS,2. When lactose is present,A small amount of a sugar allolactose is formed within the bacterial cell. This fits onto the re
9、pressor protein at another active site (allosteric site) This causes the repressor protein to change its shape (a conformational change). It can no longer sit on the operator site. RNA polymerase can now reach its promoter site, 2007 Paul Billiet ODWS,3. When both glucose and lactose are present,Thi
10、s explains how the lac operon is transcribed only when lactose is present. BUT this does not explain why the operon is not transcribed when both glucose and lactose are present., 2007 Paul Billiet ODWS,When glucose and lactose are present RNA polymerase can sit on the promoter site but it is unstabl
11、e and it keeps falling off,4. When glucose is absent and lactose is present,Another protein is needed, an activator protein. This stabilises RNA polymerase. The activator protein only works when glucose is absent In this way E. coli only makes enzymes to metabolise other sugars in the absence of glucose, 2007 Paul Billiet ODWS,Summary, 2007 Paul Billiet ODWS,
