Question

Predict the level of transcription of the trp operon genes (low or high) in the presence of low levels of tryptophan in Escherichia coli cells bearing an in-frame deletion of the two trp codons that are usually found in the leader peptide. Explain your answer.

Answer 1

Trp operon is a repressible operon which means that the transcription or expression of the operon is repressed i.e. the presence of tryptophan in the medium represses the expression of cistron in the trp operon which actually encodes for enzyme tryptophan synthetase. It is important to note here that expression of the trp genes is determined by the absence of tryptophan. Low levels of tryptophan are sensed by the bacteria and the operon is usually upregulated to express the required genes so that the optimum levels of tryptophan might be reached in the cell. However, the integrity of the cistronic genes is mandatory for complete transcription of the cistron.

Thus, according to the information, the tryptophan concentration in the medium is low which will induce the cells to mildly up-regulate the operon. Secondarily, the cistron suffers from an in-frame deletion of leader peptide which promotes post-translational modification in the nascent polypeptide. This fact will actually not prevent transcription but the expression of the desired genes will be partly functional only. Thus, this transcription will not result in generation of a functional protein.

Hence, as per the conditions, the tryptophan operon would be mildly upregulated but the transcriptional products would not be functional.