TopicPage for thrABC 5'UTR of Escherichia coli K-12
| ||Long 5' UTRs         |
The thrABC operon, sometime referred to as the thrLABC operon, has a regulatory 5' UTR that contain a leader peptide and an attenuator (premature transcription terminator).
The ThrL leader peptide does not have a known function as a peptide product but rather serves as an indicator of charged threonine-tRNA and isoleucyl-tRNA deficiency (threonine/isoleucine starvation) by stalling at the eight Thr codons and four Ile codons in the 21 amino acid leader peptide (Freundlich, 1963; Gardner, 1979; Lynn, 1987). This stalling blocks the formation of a preemptor stem-loop that shares a stem strand with the attenuator, which in threonine/isoleucine sufficiency forms an alternative stem-loop that does not terminate transcription but does preempt the formation of the attenuator stem-loop, allowing expression of the thrABC operon and biosyntheis of threonine. The efficiency of termination at the attenuator is 90% (Lynn, 1982).
The transcription start point for the thrLABC mRNA was determined by direct RNA sequencing indicating that the 5' UTR has two closely spaced transcription start points: 5'pppA at -189 and 5'pppG at -191 relative to the ATG of thrA; only P32 gamma-labeled ATP and GTP could initiate leader transcripts in vitro; the termination points of the attenuated leader transcript was also mapped using RNase T1 digestion to -29 to -27, corresponding to U7-U9 of the trailing Us of the predicted Rho-independent transcription terminator, with the majority of the attenuated transcripts ending at -29 (Gardner, 1982). EcoGene uses the second of these starts (the A start at -189) because it is highlighted by the authors in a figure; EcoGene annotates this as initiated using promoter P1 even though it appears to be two slightly overlapping promoters. An additional transcription start was mapped to -223, 34 bp upstream of P1; the tandem P1 start sites were re-mapped as one start, a C at -190, but this is unlikely since previous work indicated that gamma-labeled CTP could not initiate P1 transcripts in vitro (Gardner, 1982; Conway, 2014). EcoGene designates the -223 promoter as P2 because it was not detected in the earlier studies. Conway et al. designates the -223 start (P2) as from the primary (P) (most upstream) promoter and the -191 start (P1) as being made from a secondary (S) promoter.
Bibliography (9 total) : Review Only   Up
Conway T, Creecy JP, Maddox SM, Grissom JE, Conkle TL, Shadid TM, Teramoto J, San Miguel P, Shimada T, Ishihama A, Mori H, Wanner BL (2014) Unprecedented high-resolution view of bacterial operon architecture revealed by RNA sequencing. MBio 5:e01442-14