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RNA-Based Regulation | Boundless Microbiology
Repression of the trp operon is sensitive to slight to moderate decreases in intracellular tryptophan concentration. In contrast, attenuation is designed to allow transcription of the structural genes of the operon only when there is nearly complete depletion of the pool of charged tRNATrp. This occurs under two conditions; when the supply of intracellular tryptophan is grossly insufficient and/or when the demand for Trp-tRNATrp greatly exceeds its availability. As we shall describe, the leader peptide coding region, trpL, of the trp operon of these two bacterial species has only two tryptophan codons, which are adjacent. This design allows relief of termination only when the intracellular concentration of Trp-tRNATrp is so low that rapid translation of both of these tryptophan codons cannot occur.
Transcription of the five major structural genes of the trp operon of E. coli and S. enterica is regulated by both repression and transcription attenuation. As previously mentioned, these regulatory mechanisms allow a response, respectively, to changes in the intracellular concentrations of tryptophan and tryptophan-charged tRNATrp, Trp-tRNATrp. The intracellular tryptophan concentration is determined by several events: tryptophan import from the cell’s environment, tryptophan produced internally by biosynthesis, and the rate of use of tryptophan during protein synthesis. The concentration of charged tRNATrp also depends on several factors: the intracellular concentrations of tryptophan, tRNATrp, and tryptophanyl-tRNA synthetase (see Aminoacyl tRNA Synthetases), and the overall rate of protein synthesis. It is apparent that the regulatory strategies that control transcription of the trp operon of E. coli and S. enterica were designed to adjust the rate of synthesis of this amino acid in response to all extracellular and intracellular events that alter the availability of tryptophan and Trp-tRNATrp for protein synthesis. Feedback inhibition of anthranilate synthase provides an additional important level of regulation by tryptophan by controlling entry of chorismate into the biosynthetic pathway.
Attenuation of the Tryptophan Operon: ..
Feedback inhibition of anthranilate synthase can have a significant impact on the extent of repression. Thus, when E. coli is synthesizing the tryptophan it needs for growth and thus, repression is incomplete, feedback inhibition by the available tryptophan partially inhibits anthranilate synthase activity. This results in the production of slightly higher levels of the tryptophan biosynthetic enzymes, which is achieved by partial relief of repression. Feedback inhibition is a particularly effective regulatory mechanism because it reduces tryptophan synthesis instantaneously, whereas repression and attenuation have delayed effects. By employing complementary regulatory mechanisms, E. coli and S. enterica exploit the advantages that can be derived from different means of sensing and responding to tryptophan availability for protein synthesis.
The principal trp promoter, trpP1, is a relatively strong promoter; in tryptophan-deficient cells, RNA polymerase can initiate transcription at this promoter every 4-5 seconds (3). Inasmuch as a transcribing polymerase molecule could reach the attenuator region within a few seconds, it is essential that the decision be made quickly whether or not to terminate transcription at the attenuator. Because this decision is based on the transcript position of the ribosome that is attempting synthesis of the leader peptide, it is essential to synchronize translation of the leader peptide coding region with transcription of the leader region. Synchronization is achieved by features of the nucleotide sequence of the leader transcript; other features of the transcript determine the appropriate transcription termination response, which depends on the cellular level of charged tRNATrp (3).
BIOL 2153 Smith Exam 4 Flashcards | Quizlet
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