Operon activation

The cytoplasm represents the rest of the cell and it contains the products produced via the activation of any and all of the operons (in the genome). The state of the cell (or its type) is determined at any time during development by the mix of protein products available within its cytoplasm. In addition, the proteins within the cytoplasm of a cell determine the next set of operon activations in the genome. 

The most significant difference between the action of the trp and lac repressors relates to the function of the small-molecule effector. In the case of lac, the effector mol-ecule allolactose acts as an antirepressor (inducer), causing release of repressor from the operator in the case of trp, the effector molecule L-tryptophan acts as a corepressor, stimulating the binding of repressor to the operator. It should be obvious that the difference in action of these small-molecule effectors, the concentrations of which dictate the level of operon activity, is well suited to the different metabolic needs of the cell satisfied by the two operons. 

This targeted domain replacement of srf AC was successfully carried out for three bacterial domains of the grs operon activating Phe, Om. and Leu as well as for the Val-and Cys-activating domains of the fungal aevA gene. Surfactin derivatives produced by the chimeric synthetases were extracted from the cultured broth of different recombinant strains, examined for their hemolytic activity, and analyzed by infrared spectra and mass spectrometry. These studies clearly confirmed the identity of five novel Upopeptides that were produced by the selective domain exchange within the srf A operon. The surfactin isomers were found to carry the desired amino acid residue, as was expected from the corresponding domain replacement. 

The regulator gene, i, is located near the operon. It encodes the lac repressor protein which interacts specifically with the operator. Deletions of the i gene result in constitutivity— that is, very high levels of operon activity in the absence of inducer. 

Beckwith, J.R. Restoration of operon activity by suppressors. Biochim. biophys. Acta (Amst.) 76, 162-164 (1963). 

The trp repressor controls the operon for the synthesis of L-tryptophan in Escherichia coli by a simple negative feedback loop. In the absence of L-tryptophan, the repressor is inactive, the operon is switched on and the enzymes which synthesize L-tryptophan are produced. As the concentration of L-tryptophan increases, it binds to the repressor and converts it to an active form so that it can bind to the operator region and switch off the gene. 

Soft rot spreading depends on the efficiency of the iron uptake pathway mediated by the siderophore chrysobactin. Biosynthesis of the ferrichrysobactin outer membrane receptor (Fct) and of the chrysobactin precursor, i.e. the activated form of 2,3-dihydroxybenzoic acid, are encoded by an operon,/cr ebsCEBA [3]. 

Wells JE, PB Hylemon (2000) Identification and characterization of a bile acid 7a-dehydroxylation operon in Clostridium sp. strain TO-931, a highly active 7a-dehydroxylating strain isolated from human feces. Appl Environ Microbiol 66 1107-1113. 

The mandelate pathway in Pseudomonas putida involves successive oxidation to benzoyl formate and benzoate, which is further metabolized via catechol and the 3-ketoadipate pathway (Figure 8.35a) (Hegeman 1966). Both enantiomers of mandelate were degraded through the activity of a mandelate racemase (Hegeman 1966), and the racemase (mdlA) is encoded in an operon that includes the next two enzymes in the pathway—5-mandel-ate dehydrogenase (mdlB) and benzoylformate decarboxylase (mdlC) (Tsou et al. 1990). 

In microbes without a permeability barrier, or when the barrier fails, a mechanism must be in place to export metals from the cytoplasm. These active transport systems involve energy-dependent, membrane-bound efflux pumps that can be encoded by either chromosomal- or plasmid-borne genes. Active transport is the most well-studied metal resistance mechanism. Some of these include the ars operon for exporting arsenic from E. coli, the cad system for exporting cadmium from Staphylococcus aureus, and the cop operon for removing excess copper from Enterococcus hiraeP i9A0... 

Some metals can be converted to a less toxic form through enzyme detoxification. The most well-described example of this mechanism is the mercury resistance system, which occurs in S. aureus,43 Bacillus sp.,44 E. coli,45 Streptomyces lividans,46 and Thiobacillus ferrooxidans 47 The mer operon in these bacteria includes two different metal resistance mechanisms.48 MerA employs an enzyme detoxification approach as it encodes a mercury reductase, which converts the divalent mercury cation into elemental mercury 49 Elemental mercury is more stable and less toxic than the divalent cation. Other genes in the operon encode membrane proteins that are involved in the active transport of elemental mercury out of the cell.50 52... 

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