Regulation of lac and ara Operons Quiz

It acts as the inducer of the lac operon

Each homodimer of the Lac repressor can bind one operator sequence

It converts lactose to galactose and glucose, and a small amount of allolactose

It represses the operon by binding the operator region

It cannot bind the operator, allowing transcription from the trp operon to be initiated

It binds to tryptophan, serving as an effector and blocking transcription from the trp operon

They can base-pair to form a stem-and-loop structure, acting as a transcriptional terminator

Sequences 3 & 4 form a hairpin, RNA Pol stalls, and transcription is terminated

The amount of free tryptophan available for protein synthesis

Up to ~70 fold by the repressor system alone

It can repress the expression of the trp genes up to ~700 fold

It has 14 amino acids, two of which are Trp

Mediates lactose entry into the cell

Acts like allolactose, inducing transcription

Recognizes X-Gal as a substrate, aiding in blue-white selection

Blocks expression of lac genes when glucose is available

Through cAMP and CRP-mediated regulation

By AraC protein, arabinose, and cAMP/CRP

AraC binding arabinose

Modifies toxic galactosides imported with lactose

lacY protein

Acts as a positive regulator, influencing gene expression

AraC protein

LacZ recognizing X-Gal as a substrate

True

True

True

Trp repressor = Forms a complex with tryptophan to bind the operator and block transcription Attenuation sequence 3 and 4 = Can base-pair to form a stem-and-loop structure which acts as a transcriptional terminator Leader peptide = Translated from sequence 1 and contains two Trp codons tRNATrp and ribosomes = Affect the formation of a hairpin structure in the attenuation process

High [tryptophan] = Results in the formation of a Trp-repressor complex that binds the operator and terminates transcription Low Trp [tRNA] and ribosome stalling = Allows sequences 2 and 3 to associate and form a hairpin, preventing transcriptional termination High [tRNATrp] = Leads to rapid translation through Trp codons and formation of a transcriptional terminator Low [tryptophan] = Causes ribosome stalling on Trp codons of sequence 1 and allows the formation of a hairpin structure

Repressor system alone = Can repress the trp operon expression by up to ~70 fold in the absence of tryptophan Repressor-attenuation system combined = Can achieve up to ~700 fold repression of the trp operon expression in the absence of tryptophan Free tryptophan availability = Determines the level at which the trp operon is transcribed Attenuation process = Involves the formation of a stem-and-loop structure by sequences 3 and 4, acting as a transcriptional terminator

The trp operon is regulated by a repressor system and attenuation. In the absence of tryptophan, the Trp repressor cannot bind the operator, leading to initiation of transcription and tryptophan production. However, when tryptophan is abundant, it serves as an effector for the Trp repressor, leading to the formation of a Trp-repressor complex that blocks transcription. Additionally, attenuation occurs through the regulation of transcription that is in progress, where the level of free tryptophan determines the transcription level of the trp operon.

Attenuation of the trp operon occurs through the regulation of transcription that is in progress. The trp mRNA includes a 5’ leader sequence containing four regulatory regions. When tryptophan levels are high, tRNATrp is also high, and ribosomes translate quickly through Trp codons, leading to the formation of a hairpin and termination of transcription. On the other hand, in the absence of tryptophan, tRNA levels are low, and ribosomes stall on Trp codons, allowing the formation of a different hairpin that does not terminate transcription. Therefore, the level of free tryptophan determines the expression level of the trp operon.

In the absence of tryptophan, the expression of the trp genes can be repressed by the repressor system alone up to ~70 fold, and up to ~700 fold by the repressor-attenuation system combined. The Trp repressor system prevents the binding of the Trp repressor to the operator, allowing transcription initiation and tryptophan production. However, the combined repressor-attenuation system further enhances the repression of trp gene expression by regulating transcription in progress, based on the level of free tryptophan available for protein synthesis.