Question

Expression of the trp operon is regulated by the level of free tryptophan in the cell both through repressor action and attenuation. This chart shows the percent expression of the trpoperon in the presence and absence of tryptophan for wild-type (trpR+ or trp+) and repressor mutants (trpR-).

Use this information to determine the levels of expression for the following genotypes and conditions.

Rank the genotypes and conditions from highest to lowest level of trp operon expression.

Please use information below to solve. Thank you!

Answer 1

Ans. (Highest - Lowest) - trp+/tryptophan absent - trp L-/tryptophan Present - trp R-/tryptophan present - trp+/tryptophan Present - trp P- / tryptophan absent

trp+/tryptophan present - trp+ indicates it is wild type, for wild type in the presence of tryptophan, tryptophan acting as corepressor and activating the repressor and this repressor will bind at the operator site and stop the transcription of trp Genes and operon termed as switch OFF. some base level of gene expression will be observed though the Repressor stops the transcriptional process. so

trp+/tryptophan Present - show basal level of Gene expression.

trp+/trptophan absent - trp+ indicates it is wild type, in the wild type when trptophan is absent, the trptophan operon activates or operon ON and repressor releases from the operator site and RNA polymerase binds at the promoter and starts the transcription of tryptophan genes.

trp+/tryptophan absent - show maximum level of gene expression

trpR-/trptophan present - trpR- represents it is mutated type. in the wild type usually when the presence of tryptophan repressor binds to operator and stops the trp operon expression but in the mutated form the repressor can not form strong bonding at the promoter site, so we can observe trp operon expression of 33% (as given in the table) compare to wild type.

trp R-/tryptophan present - 33% gene expression

trp P- /tryptophan absent - trp P codes for the Promoter sequence to which RNA polymerase bind and starts the transcription of trp genes. trp P- means the operon does not have promoter sequence, when there is no promoter site the RNA polymerase can not initiate the transcription process of trp genes even in the absence of tryptophan. so there is no expression of trp operon.

trp P- / tryptophan absent - no expression of trp Operon

trp L-/ tryptophan present - trp L- is another mutated form, in the wild type trpL region codes sequence that when transcribed into mRNA able to form stem loop structure which causes transcription stops before the trp genes can be transcribed. but in mutated form trp L- cannot transcribed into mRNA and cannot form the stem loop structure. in that case even in the presence of tryptophan the the expression is not regulated by trp L.

trp L-/tryptophan Present - high level expression of trp operon