Bacterial operon repression

AraC-protein X chimeras activate transcription of the araBAD operon (left). Tandom LexA operators have been engineered between the transcription and translation start sites of the araBAD operon. The interaction of the AraC-X and LexA-Y proteins causes substantial DNA bending and repression of transcription (right). C. A bacterial activator system developed by Hochschild [55], A,-Repressor was found to activate transcription simply by stabilizing the RNA Pol-promoter complex. Thus, a protein X-z.-repressor fusion and a protein Y fusion to the N-terminal domain of the a-subunit of RNA Pol activate transcription of a reporter gene if protein X and Y interact. 

B. cAMP is involved in catabolite repression. Bacterial cells preferentially use glucose. When glucose is low, cAMP rises. cAMP binds to a protein and complexes near the lac promoter region, facilitating binding of RNA polymerase. Lactose must be present to inactivate the repressor, so that the operon may be expressed. 

In the presence of nitrate, formate dehydrogenase-N (FDH-N) and nitrate reductase are induced (8). As a result, the formate is consumed preferentially by FDH-N due to its inherent high affinity for formate versus FDH-H. The Km value for formate of FDH-N is 0.12 mM, while that of FDH-H is 26 mM. The FHL system is induced by the presence of formate, however, formate consumption by an FDH-N - nitrate reductase system depresses the expression level of the FHL system. Consequently, both expression of FHL and hydrogen production are repressed by the presence of nitrate in the medium. The removal of nitrate from waste water is not a practical process therefore a bacterial strain capable of hydrogen production even in the presence of nitrate is favorable. The subunits of nitrate reductase are encoded in the fdn operon. The a-subunit of nitrate reductase is encoded in the... 

Much of the experimental evidence on which the operon theory is based was obtained by studying the induction and repression of bacterial enzymes. Induction is indicated by an increase in enzyme activity, and repression by a decrease in that activity. The inducer is usually a substrate of the enzyme the repressor is usually a product of the enzyme reaction. The changes in enzyme activities result from alterations in the rate of protein synthesis. 

The conclusion to be drawn from the results of this experimental work is that if a series of reactions catalyzed by several enzymes in succession is necessary for a particular biochemical process, the genes determining them show a tendency to be arranged in the bacterial chromosome in a single cluster, the operon. The operon can be in an active or repressed state depending on the presence or absence of substrate (inducer). 

Possible Nature of Genetic Repressors and Mechanisms of Repression of Operons and Cistrons of the Bacterial Chromosome... 

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