Studies on the synthesis of the metabolite sauron reveal the involvement of an o
ID: 148338 • Letter: S
Question
Studies on the synthesis of the metabolite sauron reveal the involvement of an operon that you call frodo. You believe frodo consists of a structural gene responsible for synthesis of sauron, an operator, a promoter, and a repressor, genes you arbitrarily label A,B,C, and D. However, you don't know which gene is represented by A,B.C, or D. You turn to your panel of E. coli mutants cultured in the presence and absence of sauron and assay for the activity of the structural gene involved in its synthesis. In addition, you perform conjugation experiments with F' plasmids obtained from wild type cells thereby creating partial diploids among your mutant strains and again, monitor the activity of the enzyme. From the table of expression of shown below, define the functions of A,B,C, and D (if that is possible given the data provided) and also state whether the operon is inducible or repressible. Give your reasons for the assignments you give to the four genes within operon frodo. Genotvpe A+B+C+D+ A-B+C+D+ A+B-C+D+ A+B+C-D+ A+B+C+D- A-B+C+D+/F' A+B+C+D+ A+B-C+D+/F' A+B+C+D+ Active Enzyme Active Enzyme No Enzyme Inactive Enzyme Active Enzyme Active Enzyme Active Enzyme No Enzyme Active Enzyme No Enzyme No Enzyme Active Enzyme Active Enzyme No Enzyme A+B+C-D+/FA+B+C++ Active and Inactive Enzyme mix No Enzyme No Enzyme A+B+C+D-/F' A+B+C+D+ Active Enzyme Please provide your answers as a Wordfile placed in the Drop Box before the start of Unit 3. Remember to clearly label your file so that I know who it is from.Explanation / Answer
The functions of the 4 genes are:
A - Operator
B - Promotor
C - Structural gene
D - Repressor
The operon is inducible.
Explanation:
Inducible operons are those that are switched on when their corresonding metabolite is present. We see that when all 4 genes are present in their functional form, the active enzyme is produced i.e., the operon is switched on when sauron is present. Hence, by definition the operon is inducible.
The functions of the 4 components of the operon are as follows:
Promotor - It is the site to which RNA polymerase binds during intiation of transcription. If the promotor is mutated, transcription is not initiated and no enzyme is produced.
Operator - It is the site to which repressor binds. When the repressor binds to it, transcription stops and the enzyme is not produced. If operator is mutated, repressor cannot bind to it, so transcription takes place and active enzyme is produced irrespective of whether repressor is present or absent.
Repressor - It inhibits the production of enzyme by preventing gene expression. If repressor is mutated, it is not effective in binding to the operator and cannot prevent gene expression, so active enzyme is produced.
Structural gene - It contains the genetic code for the enzyme. If structural gene is mutated, enzyme is produced in its inactive form or not produced at all.
Let us consider the unconjugated case first.
Let us first look at C. We next see that when C is mutated, inactive enzyme is produced when sauron is present. This means that C is the structural gene. No enzyme is produced when sauron is absent because the operon is inducible.
We next see that when B is mutated, no enzyme is produced at all in both sauron present and absent cases, so B cannot be the repressor or the operator. So B has to be the promotor.
This leaves the operator and the repressor.
To determine which of A and D is the operator and which the repressor, let us consider the conjugation experiment results. Remember that for the frodo operon, repressor is expressed when sauron is absent (since it is inducible).
We see that when E. coli with mutant A is conjugated with E. coli with wild type, active enzyme is produced both when sauron is present and when it is absent. If A were the repressor, no enzyme would have been produced when sauron is absent. This is because the expression of wildtype A would compensate for the mutated A and would be sufficient to repress the activity of the operon. So, A is not the repressor. So it has to be the operator. This consequently means that D is the repressor. Note that when mutant D is present along with wildtype D no enzyme is produced in the absence of sauron, which matches with our reasoning explained above.