https://orcid.org/0000-0003-2300-9238
Figures & data
Figure 1. Domains and structures of SPT5/NusG in different organisms.
(a) Schematic representation of SPT5 domain organization across the three domains of life (modified from Decker et al [Citation59]. and Zuber et al [Citation163].). NTD, N-terminal domain; NGN, NusG N-terminal domain; KOW (Kyprides, Ouzounis, Woese) domain; P, phosphorylation. (b) Cryo-EM structure of bacterial NusG bound by RNA polymerase (PDB: 6C6U). (c) Cryo-EM structure of human SPT5 bound by SPT4, NELF, and paused Pol II (PDB: 6GML).
Figure 2. Functions of SPT5 and NELF in stabilizing promoter-proximal Pol II pausing.
(a) Pol II undergoes two sequential pausing steps at promoters. Under normal condition, SPT5 and NELF associate with, and stabilize, Pol II at the first pausing site. SPT5 contributes to Pol II protein stability by preventing RPB1 degradation. The transition to the second pausing site is accompanied by the dissociation of NELF and the binding of additional pausing factors such as PAF1C, while SPT5 is retained. (b) Loss of NELF induces the release of Pol II from the first to the second pausing sites, but does not cause Pol II progression into productive elongation. (c) Loss of SPT5 not only induces the shift of Pol II from the first to the second pausing sites, presumably due to the impaired NELF recruitment, but also leads to proteasome-mediated RPB1 degradation that is independent of NELF.
Figure 3. Schematic of SPT5 function in regulating pause release.
(a) At the first pausing sites, SPT5 directly interacts with NELF and facilitates its optimal association with Pol II. Upon the recruitment and activation of P-TEFb or P-TEFb-containing complexes such as SEC, NELF is phosphorylated and dissociates, leading to Pol II release to the second pausing sites. P-TEFb-mediated phosphorylation of Pol II CTD and SPT5 KOWx-4/5-linker together induce the transition of the transcription machinery into an elongation conformation. (b) The regulation of phosphorylation status of Pol II CTD (left) and SPT5 (right) through the balance between kinases and phosphatases. P-TEFb or P-TEFb-containing complexes are responsible for Pol II and SPT5 phosphorylation. CTR1 and S666 of SPT5 can be dephosphorylated by the INTAC phosphatase module and by PP4.
Figure 4. Schematic of SPT5 function in elongation-termination transition.
During productive elongation, SPT5 is hyperphosphorylated, especially at its CTR1 domain. At this stage, P-TEFb not only phosphorylates SPT5, but also directly targets PP1 and PP4 to inhibit their enzymatic activity. When transitioning into termination, PP1 and PP2A catalyze the dephosphorylation of SPT5 CTR1 and Pol II CTD, leading to decelerated transcription that is targeted by termination factors. In the termination zone, the dephosphorylated SPT5 stabilizes the transcription machinery and prevents unscheduled Pol II progression. Therefore, SPT5 loss induces widespread transcriptional readthrough.
