Regulation of Gene Expression

Bacteria (E. coli): number one choice for fuel is glucose but can also breakdown lactose into glucose and galactose and convert arabinose into ribulose-5-phosphate

Operon: allows many bacterial genes to be transcribed and regualted together

Negative Regulation: involves repressors, often small molecules and/or proteins that reduce RNA Polymerase-Promoter interactions.

Positive Regulation: involves activators which improve contacts between RNA Polymerase and its promoters.

 

Lactose Operon (lac Operon)

When glucose is abundant and lactose is not bacterial cells make very low levels of the lactose metabolism proteins.

Genes:

lac Z: b-galactosidase, catalyzes the hydrolysis of lactose into Glucose and Galactose

lac Y: permease, involved in lactose transport

lac A: thiogalactoside transacetylase

Negative Control: lac Repressor, decreases expression of lac Operon by binding to the lac Operator

Positive Control: c-AMP-CAP complex assists the RNA Polymerase binding the the lac Promoter and transcribes the lac Operon

 

Arabinose Operon (ara Operon)

Genes produce three enzymes from ara A, ara B and ara D which convert arabinose into ribulose-5-phosphate. A fourth gene ara C produces a regulatory factor called C-protein.

Negative Control: in the absence of arabinose the C-protein binds to regulatory sites forming a loop which blocks transcription of ara Operon.

Positive Control: the C-protein binds to arabinose and then to regulatory sites forming a different kind of loop which allows transcription of ara Operon.

 

Tryptophan Operon (trp Operon)

Genes produce five enzymes from trp E, trp D, trp C, trp B and trp A for the synthesis of tryptophan.

Negative Corepression: the trp R gene produces trp Repressor, a protein which must first bind to Trp (its co-repressor), before binding to the trp Operon and blocking transcription.

Attenuation: At low levels of trp full length mRNA is made leading to enhanced transcription, at high levels of Trp transcription of the trp Operon is prematurely halted. Attenuation works by coupling transcription to translation and is a suitable regulatory mechanism for amino acids. Prokaryotic mRNA does not require processing and since prokaryotes have no nucleus translation of mRNA can start before transcription is complete.

Humans:

1.      List the three types of regulation of Eukaryotic gene expression.

 

2.      List the three ways gene content or location can be altered.

 

 

3.      The three methods of regulation through Individual Genes are: Trans-acting Elements, Cis-acting Elements and Response Elements.

 

 

a.       Under Trans-acting elements:

                                                              i.      List the three types of regulatory proteins generally associated with DNA Binding Domains.

 

b.      Under Cis-acting elements:

                                                              i.      List the two major types of Cis-acting Elements.

 

4.      Describe polyadenylation. What is its function? How does the length of a poly A tail influence the stability of mRNA?

 

 

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