2*3 antiterminator

Three possible hairpin loops can form in this RNA—the 1-2 pause structure, the 3-4 terminator, or the 2 3 antiterminator. Transcription begins normally and proceeds until position 92, at which point the T2 pause structure can form. This causes RNA polymerase to pause in its RNA synthesis. A ribosome begins to translate the leader sequence, which releases the RNA polymerase from its pause and allows transcription to resume. The ribosome follows closely behind the RNA polymerase shown in Figure 11.16. The ribosome stops over the UGA stop codon of the mRNA, which prevents the 2 3 anti terminator hairpin from forming and allows instead the 3-4 terminator hairpin to form. This hairpin has the series of uracils characteristic of rho-independent termination. The RNA polymerase ceases transcription when this terminator structure forms. 

The Dormant Prophage State of A Is Maintained by a Phage-Encoded Repressor Events That Follow Infection of Escherichia coli by Bacteriophage A Can Lead to Lysis or Lysogeny The N Protein Is an Antiterminator That Results in Extension of Early Transcripts Another Antiterminator, the Q Protein, Is the Key to Late Transcription... 

The N Protein Is an Antiterminator that Results in Extension of Early Transcripts... 

Roberts, J. W., RNA and protein elements of E. coli and A transcription antitermination complexes. Cell 72 653-656, 1993. 

Lazinski, D., Grzadzielska, E. Das, A. (1989). Sequence-specific recognition of RNA hairpins by bacteriophage antiterminators requires a conserved arginine-rich motif. Cell 59, 207-18. 

Figure 1 Generalized model for sensing regulatory effectors by nascent mRNA leader transcripts. Transcription attenuation mechanisms have been identified in which the nascent transcript interacts with a translating 70S ribosome, a protein, an RNA molecule or a small metabolite, (a) Binding of the effector molecule promotes transcription termination, (b) Binding of the effector molecule promotes transcription readthrough (antitermination). See text for details.

TrpR, which is a DNA binding repressor protein, regulates transcription initiation of the E. coli trpEDCBA operon. Under tryptophan limiting conditions, TrpR represses transcription initiation, whereas repression is relieved in the presence of excess tryptophan. Once transcription initiates the elongating transcription complex is subject to control by transcription attenuation (reviewed in References 5 and 6). The leader transcript can form three RNA secondary structures that are referred to as the pause hairpin, the antiterminator structure, and an intrinsic terminator hairpin. Because the antiterminator shares nucleotides in common with the terminator, their formation is mutually exclusive. The pause hairpin has two additional roles in this transcription attenuation mechanism it serves as an anti-antiterminator stmc-ture that prevents antiterminator formation, and it codes for a leader peptide. A model of the E. coli trp operon transcription attenuation mechanism is presented in Fig. 2a. 

In several bacterial species, uncharged tRNA serves as the effector molecule in controlling expression of several aminoacyl-tRNA synthetase genes and a few amino acid biosynthetic oper-ons by a conunon mechanism termed T-box antitermination. 

The most recently identified class of transcription attenuation mechanism involves direct sensing of the effector molecule by the nascent transcript (52-54). These RNA sensors control metabolically diverse pathways. As for the other attenuation and antitermination mechaiusms discussed thus far, recognition of the particular effector molecule occurs with the appropriate affinity and high specificity required for precise control of gene expression. 

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