9. In the presence of high intracellular concentrations of tryptophan, only shor
ID: 209977 • Letter: 9
Question
9. In the presence of high intracellular concentrations of tryptophan, only short transcripts of the trp operon are synthesized because of attenuation of transcription 5' to the structural genes. This is mediated by the recognition of the two Tre codons in the leader sequence. What effect would mutating these two codons to UAG stop codons have on the regulation of the operon in the presence or absence of tryptophan? Explain. (1 pt) 10. Would you expect to see attenuation in the lac operon and other operons that control the metabolism of sugars? Why or why not? (1 pt)Explanation / Answer
9. The tryptophan operon regulates the synthesis of amino acid tryptophan via five steps that require one particular enzyme. In E. coli, these enzymes are translated from a single polycistronic mRNA. A promoter, an operator, and two regions called the leader and the attenuator are present before the coding sequences. Both the leader and attenuator sequences are transcribed while a trp repressor genes (trpR) coding for a repressor is preset away some distance.
A162-base leader sequence is present between the 5’ end of trp molecule and the start codon of the trp E gene. This bases 123-150 within this attenuator region has regulatory activity. Once mRNA synthesis is initiated, the mRNA is terminated if there is excess tryptophan in the medium. Thus, a mRNA of only 40 amino acids lacking the structural genes is synthesized. The leader sequence has an AUG codon that is in-phase with a UGA stop codon. These signals encode a polypeptide of 14 amino acids. If this leader peptide is translated, then there is premature termination of mRNA. If there is less tryptophan, then there is insufficient tRNATrp. Hence, there will be a pause in translation at the try codons.
At intermediate levels of tryptophan, some transcripts terminate while some will proceed through the operon. This depends on how fast the trp codons in leader sequence are translated. The effect of nonsense mutations will be seen in intermediate levels of tryptophan. The ribosomes will stall at these codons, thereby mimicking trp starvation conditions. Hence, there will be maximum expression of the operon. The trp operon would be expressed at maximum levels in the absence of tryptophan. This is because the trp repressor is unbound. Hence, the mutant leader region allows full expression. The expression of trp genes will also be seen in presence of tryptophan. Thus, levels of tryptophan will not have any effect of expression of trp operon.
10. The lac operon is regulated by the expression of a repressor protein. When lactose is present in the medium, it will bind to the repressor protein. Hence, the repressor protein can no longer bind the operator. RNA polymerase then binds to the operator to initiate transcription. If lactose is absent, then repressor binds the operator. As a result, RNA polymerase cannot bind the operator to initiate transcription. There is no leader sequence present in lac operon or other operon that degrade sugars. Attenuation only prevents completion of transcription. There is no block to transcription. In lac operon, the lac genes are either transcribed or not at all depending on whether the sugar is present or not. There is no short sequences transcribed in any of these operons. The sugar operons are catabolic operons. Attenuation is seen only in anabolic operons, which allows the cell to produce less mRNA when the amino acid is present in the environment. Operons that metabolize sugar are expressed only when glucose is absent. When glucose is present, the operons will not be operational as it is the preferred source of energy. Hence, there is no requirement of attenuation.
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