Transcription termination has evolved to proceed through diverse mechanisms. For several classes of terminators, multiple models have been debatably proposed. Recent single-molecule studies on bacterial terminators have resolved several long-standing controversies. First, termination mode or outcome is twofold rather than single. RNA is released alone before DNA or together with DNA from RNA polymerase (RNAP), i.e. with RNA release for termination, RNAP retains on or dissociates off DNA, respectively. The concomitant release, described in textbooks, results in one-step decomposition of transcription complexes, and this ‘decomposing termination’ prevails at ρ factor-dependent terminators. Contrastingly, the sequential release was recently discovered abundantly from RNA hairpin-dependent intrinsic terminations. RNA-only release allows RNAP to diffuse on DNA in both directions and recycle for reinitiation. This ‘recycling termination’ enables one-dimensional reinitiation, which would be more expeditious than three-dimensional reinitiation by RNAP dissociated at decomposing termination. Second, while both recycling and decomposing terminations occur at a hairpin-dependent terminator, four termination mechanisms compatibly operate at a ρ-dependent terminator with ρ in alternative modes and even intrinsically without ρ. RNA-bound catch-up ρ mediates recycling termination first and decomposing termination later, while RNAP-prebound stand-by ρ invokes only decomposing termination slowly. Without ρ, decomposing termination occurs slightly and sluggishly. These four mechanisms operate on distinct timescales, providing orderly fail-safes. The stand-by mechanism is benefited by terminational pause prolongation and modulated by accompanying riboswitches more greatly than the catch-up mechanisms. Conclusively, any mechanism alone is insufficient to perfect termination, and multiple mechanisms operate compatibly to achieve maximum possible efficiency under separate controls.
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Cover Image
Cover Image
Single-molecule imaging techniques have revealed the dynamic nature of ion channels and shown that channel activity is sometimes dependent on their mobility and mechanical forces in the lipid membrane. The cover image shows a recent high-resolution cryo-EM image of the two-pore structure of the core complex of the mitochondrial outer membrane protein translocase (TOM) from the filamentous fungus
Neurospora crassa , together with a single-molecule false-color image illustrating the calcium flux through its two pores associated with conformational changes of this protein complex. The TOM core complex undergoes reversible transitions between active (high intensity pink dots), weakly active (medium intensity pink dots) and inactive (low intensity pink dots) channel states corresponding to the suspension of movement. For more information, see the article by Nussberger and colleagues (pp. 911–922) in this issue. Image provided by Shuo Wang.
Compatibility of termination mechanisms in bacterial transcription with inference on eukaryotic models
Eunho Song, Sun Han, Sungchul Hohng, Changwon Kang; Compatibility of termination mechanisms in bacterial transcription with inference on eukaryotic models. Biochem Soc Trans 24 April 2024; 52 (2): 887–897. doi: https://doi.org/10.1042/BST20231229
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