Enzyme Repression

Enzyme repression is a form of negative control (down-regulation) of bacterial transcription. This process, along with that of enzyme induction, is called negative control because a regulatory protein brings about inhibition of mRNA synthesis which leads to decreased synthesis of enzymes.

Although feedback inhibition shuts off synthesis of the end product of a pathway, it still allows some waste of energy and carbon if the cell continued to manufacture enzymes for which it has no use. It is the process of enzyme repression that prevents the synthesis of the enzymes concerned with the synthesis of that particular end product. In the case of the pathway of tryptophan biosynthesis (Figure 3), the end product of the pathway, tryptophan, serves as an effector molecule that can shutdown the synthesis of the Enzymes a, b, c, d, and e that are concerned with tryptophan biosynthesis. This results in saving of many molecules of ATP which must be expended during protein synthesis, and it conserves amino acid precursors for synthesis of other proteins. The process is slower to act than is feedback inhibition (which acts immediately) because pre-existing enzymes have to be diluted out as a result of cell division before its effects are seen.

The genes for tryptophan biosynthesis in Escherichia coli are organized on the bacterial chromosome in the tryptophan operon (trp operon). An operon is a cluster of genes that are controlled by the same elements and which are coordinately transcribed and translated. The trp operon consists of a Promoter (P) region, an Operator (O) region, an Attenuator (A) region, and the five structural genes for the enzymes involved in tryptophan biosynthesis (Trp A-E) The components of the trp operon and its control elements are described in Figure 5 and Table 2 below.

Figure 5. Genetic organization of the Trp operon and its control elements.

O = Operator specific nucleotide sequence on DNA to which an active Repressor binds.

P = Promoter specific nucleotide sequence on DNA to which RNA polymerase binds to initiate transcription. If the repressor protein binds to the operator, RNAp is prevented from binding with the promoter and initiating transcription. Therefore, none of the enzymes concerned with tryptophan biosynthesis are synthesized.

A = Attenuator DNA sequence which lies between the operator and the structural genes for trp biosynthesis. The attenuator is a barrier that RNA polymerase must traverse if it is to transcribe the genes for tryptophan biosynthesis. In the presence of trp, most RNAp molecules fall off the DNA before transcribing the trp genes. In the absence of trp, RNAp is able to traverse the attenuator region to successfully transcribe the trp genes.

Trp A, B, C, D, E = Structural genes for enzymes involved in tryptophan biosynthesis.

Trp = tryptophan end product of the biosynthetic pathway. When combined with the repressor protein the Repressor is active. Trp is called a corepressor. 

The trp operon is regulated by a regulatory gene (Trp L) associated with the trp promoter. The product of the Trp L gene is the trp Repressor, an allosteric protein which is regulated by tryptophan. The Repressor is produced constitutively in small amounts in an inactive form. When the Repressor combines with tryptophan it becomes activated and binds to the DNA of the trp operon in such a way that it blocks the transcription of the structural genes for tryptophan. Thus, in the presence of tryptophan, transcription of the genes for tryptophan biosynthesis are repressed (tryptophan is not produced), while in the absence of tryptophan, the genes for tryptophan biosynthesis can be transcribed (tryptophan is produced); See Figure 6 below.

Figure 6a. Derepression of the trp operon. In the absence of trp the inactive repressor cannot bind to the operator to block transcription. The cell must synthesize the amino acid.

Figure 6b. Repression of the trp operon. In the presence of tryptophan the trp operon is repressed because trp activates the repressor. Transcription is blocked because the active repressor binds to the DNA and prevents binding of RNA polymerase.