Question

ACTIVITY #2: Operon control of gene expression

Complete the comparison of the lac Operon and trp Operons as a means of gene regulation. 

	lac operorn	trp operon
Regulates production of
Name of Regulatory gene" and what it  produces
What binds to the operator & when does this occur:
Structural genes controlled by the operon and how their expression is controlled:
High levels of what substance affects the operon how?

Answer 1

	lac operon	trp operon
Regulates production of	The lac operon is present in E.coli and is required for metabolism of lactose. This operon is turned on and off in response to lactose. There is catabolite activator protein (CAP) binding site, a promoter site, an operator site, followed by the three genes lacy, lac Z and lac A.	Trp operon is required for synthesis of tryptophan. The operon is turned on and off in response to levels of tryptophan. There is a repressor gene trp R, followed by promoter, operator, trp L, attenuator (controls levels of tryptophan synthesis), followed by the five structural genes.
Name of regulatory gene and what it produces	lacI is regulator gene and it produces the repressor protein	trpR is regulator gene and it produces the repressor protein
What binds to operator and when does this occur	Repressor protein will bind to operator when the levels of lactose are low in the medium. When lactose levels rise, repressor binds to allolactose and cannot bind to operator.	Repressor will bind the operator when the levels of tryptophan is high. The repressor will be produced as monomers, which associate as dimers. They bind the operator and prevent tryptophan synthesis. When tryptophan levels decline, the repressor will be inactive and cannot bind to promoter.
Structural genes controlled by the operon and how their expression is controlled.	The lac Y, Z A genes are structural genes controlled by the operon. They are controlled by one operator and one repressor. When lactose is low in the medium, Repressor binds to operator and prevents binding of RNA polymerase to the promoter. Thus, expression of structural genes is inhibited. When lactose is present, Allolactose binds to the repressor, making the operator free. Catabolite activator protein CAP binds to the CAP binding site, thereby allowing RNA polymerase to bind to the promoter and initiate transcription of the cell. Lac Z produces the beta galactosidase enzyme that is catalyses the breakdown of lactose. Lac Y encodes a permease that allows entry of lactose in the cell. Lac A protein function is not known.	Structural genes trp E, trp D, trp C, trp B, and trp A, encode tryptophan synthetase. They are controlled by one operator and one repressor. When tryptophan levels are low in the cell, repressor is inactive, as tryptophan cannot bind to it. RNA polymerase to bind the promoter, which then initiates synthesis of structural genes. When tryptophan is high in the medium, it bind to repressor. The tryptophan- Repressor complex will bind to operator. Hence, RNA polymerase cannot initiate synthesis of structural genes. There is a leader peptide encode by trpL gene. When tryptophan levels are high charged trp-tRNAs are abundant. Hence, the ribosomes do not stall at the repeated trp codons in the leader peptide. Ribosomes rapidly cover regions 1 and 2 of the mRNA. As a result, stem-loop composed of regions 3 and 4 to formed, causing a transcriptional termination. This ceases the functioning of trp operon (attenuation). When tryptophan levels are low charged trp-tRNAs are low. The ribosome will stall at trp codon repeats in leader peptide. This allows step-loop 2-3 to form and prevents the transcriptional termination, as stem-loop 3-4 is not formed. Hence, entire operon to be transcribed
High levels of which substance affects the operon and how	High lactose levels will induce the operon, as allolactose will bind the repressor, making the operator free. When glucose is present in the high levels medium, glucose inhibits intracellular cAMP production. cAMP is required for binding of CAP to DNA. Hence, when glucose is present in the medium, lac operon is inhibited.	When tryptophan is high, it binds to the repressor, altering its conformation. This complex will bind to the operator and prevent Synthesis of structural genes due to block to RNA polymerase. Tryptophan represses the operon.