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E.coli has a high level of tryptophan. E. coli inhibits the expression of trp operon genes (stops transcription). This is known as repressible gene regulation.
Trp operon acts as a negative repressible feedback system. The TRpR gene is upstream from the promoter and acts as a repressor of the Trp operon. trpR is continuously expressed at a low rate. This creates a monomer of repressor protein, which is inactive.
These monomers, when tryptophan is present in the system, combine to form tetramers. This causes a conformation in the repressor that allows it to bind to the operator. It prevents the RNA Polymerase from binding and transcribe the operon. This is why tryptophan cannot be produced from its precursor. If tryptophan is not present, the repressor will be in an inactive conformation and can’t bind to the operator. Therefore, transcription continues to produce tryptophan.
Low tryptophan levels will result in the O site (operator) of the Trp operon not being occupied. Repressor proteins are also not occupied due to tryptophan low levels. This will trigger transcription of the Trp operon.
AttenuatorRNA can be divided into four short sequences (1-4), which can create three distinct secondary structures: 2-3, 1-2, and 3-4. The formation of 2-3 stem loop structures acts as an antiterminator form. 3-4 stem loops work as a terminator form.
Translation is the process of producing a protein using an mRNA sequence. Ribosomes attach to mRNA and recruit tRNA molecules containing tRNA-specific amino acids. These tRNA possess anticodons which are complementary to the codon sequence of mRNA. Ribosomes create peptide bonds between amino acids and form a peptide sequence.
Below is an image of repression under high tryptophan concentrations.
Therefore, the option is
(B) correct option is
(B) genes of the 5 trp gene (TrpA-TrpE), are not transcribed.
The order of the structural genes E, D C, B, and A is probably the most important feature of the trp operon that is necessary for attenuation. The correct option
is (A) is thus obtained.