Transcription operons

More than 30 years ago Jacob and Monod introduced the Escherichia coli lac operon as a model for gene regulation. The lac repressor molecule functions as a switch, regulated by inducer molecules, which controls the synthesis of enzymes necessary for E. coli to use lactose as an energy source. In the absence of lactose the repressor binds tightly to the operator DNA preventing the synthesis of these enzymes. Conversely when lactose is present, the repressor dissociates from the operator, allowing transcription of the operon. 

CAP controls a number of operons, all of which are involved in the breakdown of sugar molecules and one of which is the lac operon. When the level of the breakdown products of lactose is low, the concentration of cyclic AMP in the cell increases and CAP is switched on, binds to its specific operators, and increases the rate of transcription of adjacent operons. 

Fig. 24.5 Insertion of a cloned insulin gene into a vector carrying a bacterial promoter. The arrow indicates the direction of transcription. If we suppose the bacterial promoter is derived from the lactose operon then transcription will be initiated only in the presence of lactose.

Martin MC, M Fernandez, DM Linares, MA Alvarez (2005) Sequencing, characterization and transcriptional analysis of the histidine decarboxylase operon on Lactobacillus buchneri. Microbiology (UK) 151 1219-1229. 

K. B. Song, J. W. Seo, and S. K. Rhee, Transcriptional analysis of levU operon encoding saccharolytic enzymes and two apparent genes involved in amino acid biosynthesis in Zymomonas mobilis, Gene, 232 (1999) 107-114. 

HTH1GNTR helix turn helix gluconate operon transcriptional repressor E(M)B 0(0) 0(0) ... 

Self, W. T., Grunden, A. M., Hasona, A., and Shanmugam, K. T. (1999). Transcriptional regulation of molybdoenzyme synthesis in Escherichia coli in response to molybdenum ModE-molybdate, a repressor of the modABCD (molybdate transport) operon is a secondary transcriptional activator for the hyc and nar operons. Microbiology 145, 41-55. 

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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