Operon Question

AlwaysLearning wrote:



Question:
An operon contains a repressor, a promoter sequence, an operator and a structural gene. The structural gene is responsible for the metabolism of molecule X. From the data below determine whether the operon is inducible or repressible. Then assign A,B,C and D to the four parts of the operon. Explain your rationale- FE= functional enzyme, NFE= nonfunctional enzyme, NE = no enzyme

Genotype X present X absent
A+B+C+D+ FE NE
A-B+C+D+ FE FE
A+B-C+D+ NE NE
A+B+C-D+ NFE NE
A+B+C+D- FE FE
A-B+C+D+/F’A+B+C+D+ FE FE
A+B-C+D+/ F’A+B+C+D+ FE NE
A+B+C-D+/ F’A+B+C+D+ FE and NFE NE
A+B+C+D-/ F’A+B+C+D+ AE NE

Please also explain the rationale properly.

Thank you.

First let’s establish that wild type phenotype is FE NE. It is also important to remember the terminology: Repressor – gene coding for repressor protein that binds the Operator sequence and, thus, represses the transcription of downstream Structural genes. In the absence of the repressor molecule, RNA polymerase can bind Promoter sequence and transcribe Structural genes that code for enzyme in question.

Now look at what happens in the mutants:

1. Mutations in the repressor and/or operator sequences result in the permanent gene activation since both act in ‘deactivating’ genes in absence of X. That is what happens in case of A-B+C+D+ FE FE and A+B+C+D- FE FE. Thus, A and D are either repressor or operator sequences.
2. Mutations in the promoter sequence should result in no enzyme production under any circumstances since RNA polymerase cannot bind. And that is what happens in case of A+B-C+D+ NE NE meaning that B is the promoter.
3. Mutations in the structural gene sequence could result in either no enzyme or nonfunctional enzyme production. Since there is A+B+C-D+ NFE NE available in this case C is the structural gene. (Nonfunctional enzyme cannot be explained by any other type of the mutation – it has to occur within the transcribed sequence)

Now let’s examine attempts or rescue. When wild type copy is introduced into the mutants you can see that C mutant is producing both functional and nonfunctional enzyme corroborating that mutation is in the coding sequence – structural gene. B mutant is rescued which is consistent with B being promoter. Finally, this test allows to distinguish between repressor and operator. Mutations in the repressor are generally recessive meaning that when an extra copy in introduced the repressor protein is produced and it reverts to the wild type phenotype as is the case of D mutant, but mutations in the operator are dominant because operator is a DNA sequence that cannot be “fixed” by an extra copy and cannot be rescued as seen in case of A mutant.

Lastly, from all this data you can see that this operon is generally repressed unless X is present, making it inducible by X.

Good luck!