37. For each of the E. coli strains that follow, indicate the effect of the geno

Question

37. For each of the E. coli strains that follow, indicate the effect of the genotype on expression of the trpE and irpC genes in the presence and absence of tryptophan. (In the wild type IR" P" O' atu trpE" trpC trpC and trpE are fully repressed in the presence of tryptophan and are fully induced in the absence of tryptophan) Provide an explanation for your prediction. (16 pts) R repressor gene; R" product cannot bind tryptophan; R product cannot bind operator; o operator: O cannot bind repressor; att attenuator: att is a deletion of the attenuator; P promoter; P is a deletion of the trp operon promoter. a. R P O' att' trpE trpC b. R P* O* att trpE" trpC c. R" P" O' att' trpE" trpC" d. R P* o" att trpE" trpC" a. No Expression of E and Cin the presence or absence of tryptophan. The promoter is deleted; hence there will be no expression of either trpE or trpC because transcription cannot occur b. of expression of both genes in the absence of tryptophan and low levels (-30%) of expression in the presence of tryptophan. This strain does not produce repressor so transcription of the structural genes is constitutive. However, the attenuator is normal and this results in partially constitutive expression of the structural genes. c. There will be a lower level of expression of trpE and trpC when tryptophan is present and high level of expression when there is no tryptophan. The repressor produced in this strain cannot bind tryptophan and thus cannot bind to the operator so transcription of the structural genes is constitutive. However, the attenuator is normal and this results in partially constitutive expression of the structural d. genes. constitutive The repressor cannot bind to the trpE will show completely expression. operator, so the expression of trpC and The attenuator is also mu so the operon will always be transcribed at the maximal level

Explanation / Answer

a. E and C will not be expressed at all in the absence or presence of tryptophan as the promoter has been deleted. Therefore, no transcription can take place for Trp E and Trp C.
b. There will be a high level of expression of both the genes in the absence of tryptophan and low levels of expression in the presence of tryptophan. The given strain is unable to produce repressor. Thus, the transcription of structural genes will be constitutive. But the constitutive expression of the structural genes will be eventually partial as the attenuator is normal.
c. The level of expression of trpE and trpC will be low when tryptophan is present and its expression will be high when tryptophan is absent. The repressor protein of this strain is unable to bind tryptophan and hence cannot bind at the operator. Thus, transcription of structural genes is constitutive. But the ultimate expression of the constitutive genes will be partial as the attenuator is normal.
d. There will be constitutive expression of trpC and trpE as the repressor cannot bind at the operator site and the attenuator is also mutated. Thus the transcriptional level of the structural genes will be maximum.

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