Genetics 6 Sections19 LessonsLifetime
The Tryptophan Operon
The Tryptophan (trp) operon contains five structural genes that code for enzymes required for its synthesis.
The trp operon is also subject to negative control.
For the repressible trp operon, negative control includes Trp itself binding to a repressor protein & facilitating the binding of the repressor to the operator: Trp is a corepressor.
The Tryptophan Operon
As repression by Trp is not always complete, unlike the lac operon, the trp operon is also regulated by a process known as attenuation.
With attenuation, transcription is initiated but is terminated well before completion.
If Trp is plentiful, transcription initiation that escaped repression by Trp, is attenuated by the formation of a hairpin (stem–loop) structure at the 5’ end the mRNA.
Transcription & translation are temporally linked in prokaryotes and, therefore, attenuation also results in the formation of a truncated, nonfunctional peptide product that is rapidly degraded.
If Trp becomes scarce, the operon is expressed.
The 5’ end of the mRNA contains two adjacent codons for Trp.
The lack of Trp causes ribosomes to stall at these codons, covering regions of the mRNA required for formation of the attenuation hairpin.
This prevents attenuation and allows transcription to continue.
Coordination of Transcription & Translation in Prokaryotes
Transcriptional regulation of mRNA production is primary in bacteria.
Regulation at the level of ribosomal RNA (rRNA) & protein synthesis also occurs and,
It plays important roles in the microbe’s ability to adapt to environmental stress.
1. Stringent Response
Regulation in response to Amino acid starvation is known as the Stringent response.
E.coli has 7 operons that synthesize the rRNA needed for ribosome assembly and each of them is regulated in response to changes in environmental conditions.
The binding of an uncharged tRNA to the A site of a ribosome triggers a series of events that leads to the production of a Polyphosphorylated Guanosine –ppGpp.
The synthesis of this unusual derivative of GDP is catalyzed by stringent factor ‘RelA’ – an enzyme physically associated with ribosomes.
Elevated levels of ppGpp result in inhibition of rRNA synthesis.
In addition to rRNA, tRNA synthesis and some mRNA synthesis (for ribosomal proteins) are also inhibited.
The synthesis of mRNAs for enzymes required for amino acid biosynthesis is not inhibited.
ppGpp appears to alter promoter selection through use of different sigma factors for RNA polymerase.
2. Regulatory Ribosomal Proteins
Operons for ribosomal proteins (r-proteins) can be inhibited by an excess of their own protein products.
For each operon, one specific r-protein functions in the repression of translation of the polycistronic mRNA from that operon.
The r-protein does so by binding to the Shine-Dalgarno (SD) sequence located on the mRNA just upstream of the first initiating AUG codon and acting as a physical impediment to the binding of the small ribosomal subunit to the SD sequence.
One r-protein thus inhibits synthesis of all the r-proteins of the operon.
This same r-protein also binds to rRNA and with a higher affinity than for mRNA.
If the concentration of rRNA falls, the r-protein then is available to bind its own mRNA and inhibit its translation.
This coordinated regulation keeps the synthesis of r-proteins in balance with the transcription of rRNA, so that each is present in appropriate amounts for the formation of ribosomes.