Question

Question 9 [11 marks / 11 minutes]
This question is about regulation of expression of the trp operon by control of attenuation. Assume there are intermediate levels of tryptophan, such that TrpR (the repressor protein) is inactive.
a)Design a mutation within the 5’ UTR of the trp operon that would give the outcome below.Explain how your mutation would result in that outcome.
i.increased expression of the operon relative to wildtype[4 marks]
ii.decreased expression of the operon relative to wildtype[4 marks]
b)Predict and explain the effect of insertion of T-A basepairs immediately after region 3, such thaturacils are transcribed. Assume this insertion does not change the ability of regions 1, 2, 3 or 4to form stem-loop structures. [3 marks]

Answer 1

Please find the answers below:

The trp operon is a repressible operon which undergoes decline in gene expression under the presence of substrate i.e. tryptophan. As a mechanism, the presence of tryptophan causes formation of a stem-loop structure in the operon which prevents gene expression. Under low tryptophan levels, the regions 2 and 3 tend to make the stem-loop while under high tryptophan levels, the regions 3 and 4 tend to make the stem-loop. Formation of a stem-loop ensures that the linear motion of RNA polymerase does not take place on the cistron, hence gene expression decreases or stops.

Thus, based upon this background, the answers can be found as below:

Answer a:

Part i) The 5' UTR is a location in the promoter region which determines the strength of binding of RNA polymerase on the cistron. The conservation of this region is highly important and thus, maintenance of the GC- base pairs in this regions would tend to increase gene expresion.

Part ii) Since the 5' UTR region represents the conserved promoter region, any change in the location or structure of this region would lead to decrease in gene expression.

Answer b: Since insertion of these extra T-A pairs do not interfere with the ability of the operon to make stem-loop structure between any of the subunits, a decrease of halt in gene expression would not take place and hence operon would be expressed as normal.