Operon suppression

Khattar, M. M. Overexpression of the hslVU operon suppresses SOS-mediated inhibition of cell division in Escherichia coli. FEBS Lett. 1997, 414, 402-404. 

Karow M. Fayet O, Georgopoulos C. The lethal phenotype caused by null mutations in the Ejcfieridiw coli JitrB gene is suppressed by mutations in the accBC operon, encoding two subunits of acetyl coenzyme A carboxylase. J Bacteriol 1992 174 7407-418. 

Some polar mutations of the lac operon have been found to be suppressed by other genes which are known to act as amber, ochre, or UGA suppressors. When z nonsense mutants are suppressed, an amino acid is inserted at the point of the mutation and the mutated enzyme activity is restored. The inserted amino acid has been found to be related to the class of suppressor employed. Suppressor genes produce mutated species of tRNA s which recognize nonsense codons, and insert amino acids at that point. For example the tyrosine tRNA of suppressor III recognizes the nonsense triplet UAG and inserts tyrosine on the growing peptide chain, thereby competing with the chain release reaction. 

The report that cAMP relieves glucose repression of iV-acetylkanamycin amidohydrolase in Streptomyces kanamyceticus, a prokaryote ( ), indicates that the repression mechanism resembles that of different catabolic enzymes in bacteria, which proceed via the inhibition of adenylate cyclase, the enzyme that converts ATP to cAMP (D 10.4). As a consequence the concentration of cAMP decreases and the transcription by RNA polymerase of operons subjected to cAMP control is inhibited (catabolite repression). In eucaryotes, however, catabolite repression could not be demonstrated. In Penicillium cyclopium for instance, glucose suppression of benzodiazepine alkaloid biosynthesis cannot be overcome by administration of cAMP or cAMP derivatives. 

Vitamin 6 2 under aerobic conditions. Glycerol dehydratase (dhaB) is highly sensitive to oxygen. Celinska (2011) discovered that the expression of the dhaB operon is suppressed if the oxygen levels are high in the culture medium. Thus it leads to the excess formation of by-products like lactic acid, acetic acid, and ethanol, respectively, and consequently a very low yield of 3-HP. It was reported that coenzyme Bj2 is not synthesized under aerobic conditions. 3-HP production is carried out imder (micro) aerobic (or) anaerobic conditions only. The introduction of vitamin B12 in recombinant strains is very difficult because more than 20 genes of vitamin B12 biosynthesis pathways need to be expressed in E. coli (Kumar et ak, 2014). 

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