Bacterial operon control

It transpires that the heat shock genes of one single bacterial species are regulated by different mechanisms. Genes and operons controlled by one particular regulator are called regulons and, if there are at least two regulons in one species induced by the same stress factor, they form a stimulon. 

Yanofsky C. Attenuation in the control of expression of bacterial operons. Nature. 1981 289 751-758. 

Fig. 14.6. Bacterial operon. A cistron encodes a single polyrpeptide chain. In bacteria, a single promoter may control transcription of an operon containing many cistrons. A single pxjlycistronic mRNA is transcribed. Its translation produces a number of polypeptide chains.

The helix-tum-helix motif is also found in many other proteins. One of these is the bacterial lac (lactose) repressor which controls the lac operon and for which... 

These two complementary systems allow the bacterial cell to metabolize lactose in response to two stimuli. Switching on the expression of the lac operon requires both the absence of glucose and the presence of lactose. This series of switches allows complex expression patterns to be built up from simple components. For this reason, the lac system is a model for other, apparently more complex, biological control systems, such as hormone action or embryonic development. 

Transcription of operons, as well as of Isolated genes, is controlled by an Interplay between RNA polymerase and specific repressor and activator proteins. In order to initiate transcription, however, E. coli RNA polymerase must be associated with one of a small number of ct (sigma) factors, which function as Initiation factors. The most common one In bacterial cells Is... 

Specific activator proteins, such as CAP in the lac operon, also control transcription of some but not all bacterial genes. These activators bind to DNA together with the RNA polymerase, stimulating transcription from a specific promoter. The DNA-bIndIng activity of an activator is modulated in response to cellular needs by the binding of specific small molecules (e.g., cAMP) that alter the conformation of the activator. 

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