Lecture 11 - Control of Gene Expression (Prokaryotes)

Question 1

Operon

Answer 1

a functioning unit of genomic DNA containing a cluster of genes under the control of a single promoter (the resulting RNA is said to be polycistronic)

Question 2

Constitutive (housekeeping) genes

Answer 2

have a relatively uniform level of transcription. This may be high or low, depending on the gene, but remains fairly constant for any given housekeeping gene.

Question 3

Repressors

Answer 3

impede access of RNA polymerase to the promoter. They reduce the expression of inducible genes in response to specific stimuli.

Question 4

Activators

Answer 4

enhance the interaction of RNA polymerase with the promoter. They increase the expression of inducible genes in response to specific stimuli.

Question 5

Binding site for transcription factors

Answer 5

major groove; palindromic DNA sites

Question 6

cAMP receptor protein (CRP)

Answer 6

When glucose is low, cAMP is high. cAMP-bound CRP enhances transcription of the lac operon 50-fold.

Question 7

Lac Repressor (LacI)

Answer 7

When lactose levels rise, allolactose binds the lac repressor and causes it to dissociate from the operator. Transcription is increased 1000-fold. (Full fxn requires that CRP is also bound)

Question 8

trp operon

Answer 8

encodes proteins required to convert chorismate to tryptophan

Question 9

trp operon is regulated in two ways

Answer 9

• tryptophan acts as an effector molecule to regulate the binding of TrpR (Trp repressor) to the operator sequence. When tryptophan levels are low, Trp repressor is released and the operon is transcribed at a high level.
• Typtophan also regulates the operon by transcriptional attenuation, in which the frequency of transcription is attenuated by available tryptophan. The process, which allows fine tuning of expression levels, is used for many amino acid biosynthetic pathways. (base pairing within the mRNA causes loop that stops transcription)