Lactose operon repressor protein

HEK-293 hGH High-Five hsp70 HSV IPTG IRES kb Lac LCR LoxP LUC MCS human embryonic kidney cells human growth hormone TM BTI-TN-5B1-4 (cell line derived from the insect Trichoplusia ni) heat shock protein 70 herpes simplex virus isopropyl 1 -thio-fi-D-galactopyranoside internal ribosomal entry site kilobases lactose operon/repressor locus control region locus of crossover of PI luciferase isolated from firefly multiple cloning site... 

Emulation by activator and repressor proteins in the lactose operon... 

In bacteria, genes that encode products with interdependent functions are often clustered in an operon, a single transcriptional unit. Transcription of the genes is generally blocked by binding of a specific repressor protein at a DNA site called an operator. Dissociation of the repressor from the operator is mediated by a specific small molecule, an inducer. These principles were first elucidated in studies of the lactose (lac) operon. The Lac repressor dissociates from the lac operator when the repressor binds to its inducer, allolactose. 

The regulation of bacterial transcription is well illustrated by the lactose operon (lac operon) of the colon bacterium Escherichia coli in which the upstream region successively (from the 5 end of the sense strand) includes a promoter (P ) for the gene (I) coding for a repressor protein (the lac repressor), a CRP binding site , the promoter for the lac operon (P), and finally an operator site (O) that prefaces the Z, Y and A structural genes of the operon ... 

The lac operon also contains a promoter and an operator that is the off and on switch for the operon. A protein called the repressor switches the operon off when it binds to the operator. When lactose is absent, the repressor is active and the operon is turned off. The operon is turned on again when allolactose (formed from lactose) inactivates the repressor by binding to it. 

The operator of the lac operon in E. coli (Fig. 94) is composed of four sites — each of them five nucleotides in length — which are separated by one-two nucleotide spacers. The synthesis of repressor proteins is controlled by a special gene known as the regulator (i). The protein repressor for each operator is constantly being synthesized, but at a very low rate. No more than 10-20 molecules are present in the cell at any time. The attachment of the activated operon protein (CAP), which is an acceptor of cAMP, and the removal of repressor protein from the DNA of the operator under the control of substrate (lactose) concentration, are both necessary for the initiation of transcription. Cyclic AMP does not associate with the activator (CAP) if its concentration is low in the cell. Hence, the operon remains in the repressed state (Blattner and Dahlberg, 1972 Blattner et al., 1972 Khesin, 1972 Bresler, J973). The termination of transcription is also determined by a special site on the DNA. 

The Lac operon is made up of three genes (designated A, Y and Z), which code for enzymes which metabolize lactose and the control element whose function is to activate transcription of the A,Y and Z genes. Normally (i.e. when there is sufficient glucose available), a protein called a repressor blocks the control element and so the A, Y and Z genes are off. 

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