Question

1. Synthesis of amino acids are homeostatically regulated such that an overproduction of a particular amino acid is prevented. Explain how this works in the trp operon of E.coli at

a) the level of the enzyme pathway and b) the level of gene regulation.

2. What is the role of a repressor of gene expression for lactose consuming bacteria? (Assume absence of glucose.)

3. Cells control their level of all proteins by regulating both synthesis and degradation. Describe the role of ubiquitination and the proteasome in regulating the level of cellular proteins.

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Answer 1

Ans:

1.

a) Feedback or end product inhibition - mechanism for the inhibition of preformed enzymes seen primarily in regulation of biosynthetic pathways e.g synthesis of amino acid tryphtophan in trp operon. These pathways involve many enzymatic steps.In this mechanism, end product is able to feed back to the first step to regulate its own production.

In this process end product of a pathway inhibits the activity of first enzyme, thus controlling production of the end product. The first enzyme in the pathway is an allosteric enzyme and allosteric site of the enzyme binds to end product (tryptophan) of the pathway which alters structure of the enzyme and therefore halts binding of RNA polmerase to promoter as a result stop transcription of trp operon genes.Pathway is turned off at adequate amounts of end product (tryptophan). When the end product is used up or disappears, transcription of trp operon genes is initiated.

b) In the absence of tryptophan- a trp repressor protein is encoded by trpR gene of the operon.Repressor is synthesiazed and forms a dimer. Repressor is unable to bind to the trp operator thus allowing the transcription of structural genes.However at high tryptophan levels, binding of repressor to operator overlaps the promoter thereby halting RNA polymerase binding to the prometer and hence stops transcription of structural genes.

2. Lac repressor is a protein that inhibits transcription of lac operon genes. Lac repressor binds to the operator and overlaps with promoter and halts RNA polymerase to the promoter thus stops expression of structural gene of lac operon. In absence of lactose, lac repressor binds tightly to the operator and preventing transcription by RNA polymerase.

In presence of lactose, lac repressor doesn't bind to operator and therefore giving access to RNA polmerase to bind to promoter for transcription of structural genes of ac operon.

3.Ubiquitin Proteasome Pathway (UPP) plays a major role in understanding of protein turnover. It consists of concerted actions of enzymes that link polypeptide, Ubiquitin (Ub), thus marking proteins for degradation. This tagging process lead recognition of proteins by 26S proteasome (multicatalytic protease complex which degrades ubiquitinated proteins to small peptides). Three enzymatic components are required to link chains of Ub onto proteins that are destined for degradation.

E1 - Ub-activating enzyme

E2s- Ub-carrier, it prepares Ub for conjugation to protein.

E3- Ub-protein ligase, it recognizes a specific protein and catalyzes transfer of activated Ub to it