The trp operon

The tryptophan operon comprises five genes trpEDCBA which when transcribed and translated enable the cell to synthesize tryptophan from glutamine and chorismate Figure 8. Tryptophan itself inhibits the first step of the pathway at the protein level but the cell also imposes regulation during both translation and transcription.

It is important to remember that the cell does not need the tryptophan synthesis enzymes when there is an abundance of tryptophan itself a contrast to the lac operon where lac- tose abundance requires that the cell induces a metabolism system. The transcription of the trp operon is thus stopped through the binding of the trp repressor complexed to tryptophan. As the levels of tryptophan fall the operon is derepressed. The difference among repression and derepression is about 70-fold much less than that of the lac operon in its on and off states and this gave a clue which there was more than one mechanism regulating trpEDCBA expression.

The number of bases between the trp promoter or operator and triplet 1 of trpE is unusually long.The 162 NT nucleotides form a Rho-independent terminator site.  When transcribed the site consists of a short GC-rich palindrome followed through eight uracil bases.

        Figure:  (a) Structure of the trp operon and function of the trp repressor. (b) The pathway of tryptophan synthesis in E. coli.

If this palindrome can hybridize to itself it forms a stem-loop structure or hair- pin that is a highly efficient transcriptional terminator allowing the RNA polymerase to synthesize only the first 140 bp of the operon. The mechanism through which these short transcripts are made is termed attenuation.

In bacteria, transcription by RNA polymerase and translation by ribosomes often occur in close proximity, with the ribosome binding to its mRNA binding site as soon as the RNA polymerase has transcribed it. To ensure that the RNA polymerase and ribosome are as close to one another as possible, the initial transcription of the first hundred bases results in the formation of a stem-loop between regions 1 and 2 of the mRNA in Figure. This causes the RNA polymerase to pause and while this takes place a ribosome loads itself onto the mRNA at the 5’ end.

When tryptophan is absent or at very low levels the ribosome starts to translate the 5’ end  of the  trp  mRNA until  it encounters two  codons coding for  tryptophan  itself UUGUUG,  region 1 in Figure. As tryptophan is at such low levels there are few tRNA- tryptophan molecules around for the  ribosome to incorporate into  the  growing peptide and therefore the ribosome is waiting a hairpin among regions 2 and  3 forms that rein- forces this pause and allows the RNA polymerase to transcribe the entire operon. Contrast this with the  situation when tryptophan is abundant: again the  RNA polymerase begins transcription pauses and  the  ribosome loads but  now  there is no  barrier to the  ribosome incorporating tryptophan so it can  move  forward unfolding the  hairpin among regions 2 and 3. This causes region 3 to be available to form another secondary structure, this time with region 4 in Figure. However, the combination of regions 3 and 4 coupled to the presence of poly U a little further downstream is a Rho-independent transcription termination signal. The RNA polymerase falls off after only having manufactured the first 140 nt of the trp mRNA. In this way the transcription of the trp operon is attenuated in the presence of tryptophan but permitted in its absence allowing the cell to conserve energy through avoiding the wasteful transcription of unneeded mRNA.

                                                  Figure:   Attenuation of the trp operon.