3.8.07 - 3.8.2007 lacI can be transcribed to mRNA which can...

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3.8.2007 lacI can be transcribed to mRNA which can be translated to make the protein the lac repressor o lac repressor has a DNA binding domin to which it binds to a sequence on DNA called the operator the function of the operator is to be a binding site for the lac repressor o what does the lac repressor do when its bound here? It stops transcription o the way the repressor stops transcription is by being physically in the way, if the repressor is bound at the operator then RNA polymerase just cannot get past it to finish the initiation of transcription, then the question becomes what regulates lacI, when is lacI produced? o lacI is constitutive constitutive chromatin heterochromatic all the time, constitutive gene is expressed all the time so there is always lac repressor in the cell o DNA binding domain of the lac repressor is specific for the binding sequence but there are actually two binding sites on the lac repressor, one is for the operator, the other one is for alolactose In order to bind to the operator the lac repressor has to conform to get the perfect fit, being able to conform its DNA binding domain to the operator causes a general shape change throughout the lac repressor which also changes the shape of the alolactose binding site If the operator is bound to lac repressor its alolactose binding site is not right In order for alolactose to bind the lac repressor, it will shift and change the shape of the DNA binding domain and it can no longer bind to the operator The lac repressor can bind to either the operator or alolactose, it cannot bind to both at the same time If both the operator and alolactose are available then the repressor binds alolactose because the repressor bound to alolactose is in a lower energy state compared to the repressor bound to the operator o Lac operon consists of an operator, the genes it controls, and their promoter P O lacZ lacy lacA __,__,_____,_____,_____ Its in the off state when the lac repressor is bound to O This is how it will be when lactose is absent, because if lactose isn’t around then the cell doesn’t need to produce beta galctosidase and permease, this is the starting state or off state If lactose becomes available, it will come into the cell via permease, what normally beta galactosidase does when it gets a hole of lactose is to split it but some percentage of the time it gets rearranged to alolactose, some of this binds to the alolactose binding site of the repressor which pulls it off of the operator Now the polycistronic mRNA can be transcribed, it can be translated to produce permease which inserts in the membrane allowing lactose to come flooding into the cell and also produces beta galactosidase which will split most of it into glucose and galactose and rearranges some of the lactose into alolactose to keep this expression going, now the operon is on Repeat: we’re in the off state with lac repressor bound to the operator expression is off…
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3.8.07 - 3.8.2007 lacI can be transcribed to mRNA which can...

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