You are studying regulation of a prokaryotic operon. Experiments show that expression of the operon is increased when levels of biosynthetic product from that pathway are low. Based on this information, what is a likely mode of regulation?

Question

A.  The product binds a repressor, allowing it to bind the operator.
B.  The product binds an activator, helping RNA polymerase to bind the promoter.
C.  The product binds a repressor, preventing it from binding the operator.
D.  The product binds to the biosynthetic enzymes, blocking them directly.

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?
· What key words does the question contain and what do they mean?

Gather Content
· What do you already know about transcriptional regulation in prokaryotes?

Consider Possibilities
· Consider the different answer options. Which can you rule out?
 Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?

Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

A.  The product binds a repressor, allowing it to bind the operator.

Clarify Question
· What is the key concept addressed by the question?
        o This question addresses transcriptional regulation.
· What type of thinking is required?
        o This question is asking you to take what you already know and apply it to this unfamiliar situation.
· What key words does the question contain and what do they mean?
        o A prokaryotic operon is a small set of adjacent genes in a bacterium that are expressed together, and usually work in the same pathway.
        o The biosynthetic product from a pathway would be the small molecule that the encoded enzymes produce as a product.

Gather Content
· What do you already know about transcriptional regulation in prokaryotes?
        o We learned about three examples of regulation in prokaryotes: lactose regulation of the lac operon, glucose regulation of the lac operon, and tryptophan’s regulation of the trp operon.
        o This example is similar to the regulation of the trp operon by tryptophan. It is an example of negative feedback by the biosynthetic product.
        o When the product is low, operon expression is high. When the product is high, operon expression is low.

Consider Possibilities
· Consider the different answer options. Which can you rule out?
        o Could this operon be regulated by the product binding an activator, helping RNA polymerase to bind the promoter? No, because then high product levels would have high expression, low product levels would have low expression.
        o Could this operon be regulated by the product binding a repressor, preventing it from binding the operator? No, because again, in that case high product levels would have high expression, low product levels would have low expression.
        o Could this operon be regulated by the product binding to the biosynthetic enzymes, blocking them directly? That would be negative feedback, but it would not affect the expression of the operon – in other words, the levels of mRNA produced for the enzyme genes.
 Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o Could this operon be regulated by the product binding a repressor, allowing it to bind the operator? Yes. This would mean that low product levels would derepress the operon, allowing higher expression.         o This is similar to the regulation of the trp operon.

Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o This question asked what is the likely mode of regulation for an operon that is controlled by negative feedback from its enzymes’ biosynthetic product.
        o The question required you to take what you already know and apply it to this unfamiliar situation.
        o Did you recognize that this is similar to the regulation of the trp operon by tryptophan?
        o Did you realize that high levels of the product probably block expression by helping a repressor to block transcription?
        o Was there an alternative explanation not listed as an option here?