The use of preformed amino acids is advantageous to a cell since catabolism of a
ID: 207995 • Letter: T
Question
The use of preformed amino acids is advantageous to a cell since catabolism of amino acids expends energy. Which of the statements regarding the E. coli trp operon are true?
1) The regulatory goal is to stimulate gene activity by turning on the synthesis of required enzymes when the end products are available.
2) The small molecule ligand tryptophan binds to a repressor, activating it.
3) The premature termination of transcription occurs if tryptophan is abundant.
4) High levels of tryptophan shut down the operon.
Explanation / Answer
Ans) B. Statements 2, 3, and 4 are true.
Tryptophan operon involved in the conversion of Chorismate to L-Tryptophan and the tryptophan operon consists of five structural genes trpE, trpD, trpC, trpB, trpA along with the leader sequence called trpL. The tryptophan operon regulated by the attenuation process. Attenuation defined as controlling termination of transcription and it is possible only in prokaryotes because of coupled nature of transcription and translation. The trp leader sequence consists of the attenuator which can be divided into the four intra-complementary segments. The 1&2, 2&3, 3&4 are self-complementary to each other. The segment 1 consist last 5 codons of leader peptide that is from 10th to 14th position.
If the segment 3&4 are hybridized the resultant secondary structure acts as transcription terminator whereas the 2&3 hybrid structure act as a transcription anti-terminator.
When the tryptophan content is high in cells tryptophanyl tRNA immediately incorporated on the surface of the ribosome at the 10th and 11th codons of the leader peptide and the movement of ribosomes not immediately inhibited (paused at trp codons) on leader mRNA. It allows the hybrid formation between the 3rd and 4th segments in leader mRNA which in turn act transcription terminator and it displaces the RNA polymerase from the transcription bubble & only leader mRNA is expressed. It shut down the tryptophan operon
When tryptophan content is low in cells the ribosome movement inhibited at the segment 1 because of the unavailability of tryptophanyl tRNA. It results in the segment 2nd and 3rd hybrid formation in leader mRNA that acts as a transcription anti-terminator so that RNA polymerase expresses the polycistronic mRNA.
Tryptophan operon also regulated by the repressor. The regulation by means of the repressor is called negative regulation. When cells with higher levels of tryptophan it binds to the repressor and activate the repressor. The try-repressor complex binds to the operator site of trp operon and inhibits its expression. Here tryptophan acts as a corepressor.
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