Tryptophan biosynthesis enzymes rectangle is a simplified representation of the
ID: 219752 • Letter: T
Question
Tryptophan biosynthesis enzymes rectangle is a simplified representation of the E. coli hromosome. within the chromosome are the trp and lac operons. Each operon contains a duster of genes. The genes within the trp operon encode proteins required for tryptophan biosynthesis. The genes within the lac operon encode proteins required for lactose breakdown. Determine which genes will be expressed when an E. coli cell is grown in this medium trp operon (contains five genes) These five genes { lac operon (contains three genes) These three genes be expressed be expressed. will not will je in environment: +lactose,-glucose, + tryptophan If glucose were added to the medium, you would expect to see in gene expression within this E. col O Type here to search A?& Ul 4 ZXExplanation / Answer
Availability in environment = +lactose, -glucose, + tryptophan
1) The five genes of trp operon will not be expressed. Trp operon is a repressible system, in which the addition of tryptophan (the effector molecule) to the E. coli environment shuts off the system because the repressors binds at the operator.
2) The three genes of lac operon will be expressed. Lac operon is an inducible system. With lactose in the cell, lactose binds to the repressor. This causes a structural change in the repressor and it loses its affinity for the operator. Thus RNA polymerase can then bind to the promoter and transcribe the structural genes. In this system lactose acts as an effector molecule.
3) If glucose were added to the medium, you would see a change in gene expression within this E.coli cell. This is because lactose is not the preferred carbohydrate source for E. coli. If lactose and glucose are present, the cell will use all of the glucose before the lac operon is turned on. This type of control is termed catabolite repression. The promoter of the lacoperon has two binding sites. One site is the location where RNA polymerase binds. The second location is the binding site for a complex between the catabolite activator protein (CAP) and cyclic AMP (cAMP). The binding of the CAP-cAMP complex to the promoter site is required for transcription of the lac operon. The presence of this complex is closely associated with the presence of glucose in the cell. As the concentration of glucose increases the amount of cAMP decreases. As the cAMP decreases, the amount of complex decreases. This decrease in the complex inactivates the promoter, and the lac operon is turned off. Because the CAP-cAMP complex is needed for transcription, the complex exerts a positive control over the expression of the lac operon.
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