Operons negatively controlled

S. Elsen, A. Colbeau, J. Chabert, P. M. Vignais (1996) The hupTUV operon is involved in the negative control of hydrogenase synthesis in Rhodobacter capsulatus. J. Bacteriol., 178 5174-5181... 

The lac operon is ordinarily subject to repression and is activated by the presence of an inducer, now known to be allolactose, D-Galp-(il— 6-d-G1c. However, in experimental work artificial inducers such as isopropyl-[5-o-thiogaIactoside (IPTG) are most often used. Jacob and Monod postulated that the free repressor protein binds to the operator. In the presence of the inducer a conformational change takes place, destroying the affinity of the repressor protein for the operator site. Thus, in the presence of inducer the operator is not blocked, and the transcription takes place. Such an operon is said to be negatively controlled and inducible. 

The galactose (gal) operon of E. coli is negatively controlled and inducible by D-galactose or D-fucose, which bind to the gal repressor. There are two overlapping promoter sites, one of which is stimulated... 

Positive and The lac operon is a good example of negative control (negative regulation) of... 

Why is control of expression of the lac operon in E. coli said to be an example of negative control ... 

Which of the following best describes the negatively controlled lactose operon in Escherichia coli ... 

The lactose operon is negatively controlled by the lactose repressor and positively controlled by which of the following ... 

Because this operon is under negative control, substance S must act as a repressor. Substance S is active as a repressor in the presence of T, so T functions as a corepressor. 

Represslble Biosynthetic Systems. In enteric bacteria the represslble systems for histidine and tryptophan biosynthesis provide examples of systems controlled by positive and negative elements, respectively (see Table IV). Since the level of histidine In the colon Is among the lowest of all amino acids, expression of the histidine biosynthetic operon In the bacterium will be In hl demand (Table III). Thus, demand theory predicts that the histidine biosynthetic operon will be positively regulated (Table II). In fact, there Is good evidence to show that this operon Is controlled primarily by an antiterminator mechanism (, ). On the other hand, the level of tryptophan In the colon Is among the hipest of the amino acids and, therefore, expressl( i of the tryptophan biosynthetic operon In the bacterium... 

---