Duchi, D. | bioRxiv Science

bioRxiv2018

The RNA polymerase clamp interconverts dynamically among three states and is stabilized in a partly closed state by ppGpp

RNA polymerase (RNAP) contains a mobile structural module, the \"clamp,\" that forms one wall of the RNAP active-center cleft and that has been linked to crucial aspects of the transcription cycle, including loading of promoter DNA into the RNAP active-center cleft, unwinding of promoter DNA, transcription elongation complex stability, transcription pausing, and transcription termination. Crystal structures and single-molecule FRET studies establish that the clamp can adopt open and closed conformational states; however, the occurrence, pathway, and kinetics of transitions between clamp states have been unclear. Using single-molecule FRET (smFRET) on surface-immobilized RNAP molecules, we show that the clamp in RNAP holoenzyme exists in three distinct conformational states: the previously defined open state, the previously defined closed state, and a previously undefined partly closed state. smFRET time-traces show dynamic transitions between open, partly closed, and closed states on the 0.1-1 second time-scale. Similar analyses of transcription initiation complexes confirm that the RNAP clamp is closed in the catalytically competent transcription initiation complex and in initial transcribing complexes (RPITC), including paused initial transcribing complexes, and show that, in these complexes, in contrast to in RNAP holoenzyme, the clamp does not interconvert between the closed state and other states. The stringent-response alarmone ppGpp selectively stabilizes the partly-closed-clamp state, inhibiting interconversion between the partly closed state and the open state. The methods of this report should allow elucidation of clamp conformation and dynamics during all phases of transcription.\n\nSIGNIFICANCE STATEMENTThe clamp forms a pincer of the RNA polymerase \"crab-claw\" structure, and adopts many conformations with poorly understood function and dynamics. By measuring distances within single surface-attached molecules, we observe directly the motions of the clamp and show that it adopts an open, a closed, and a partly closed state; the last state is stabilized by a sensor of bacterial starvation, linking the clamp conformation to the mechanisms used by bacteria to counteract stress. We also show that the clamp remains closed in many transcription steps, as well as in the presence of a specific antibiotic. Our approach can monitor clamp motions throughout transcription and offers insight on how antibiotics can stop pathogens by blocking their RNA polymerase movements.

biophysics↗

bioRxiv2017

Structural basis of transcription inhibition by fidaxomicin (lipiarmycin A3)

Fidaxomicin is an antibacterial drug in clinical use in treatment of Clostridium difficile diarrhea1-2. The active pharmaceutical ingredient of fidaxomicin, lipiarmycin A3 (Lpm)1-4, is a macrocyclic antibiotic with bactericidal activity against Gram-positive bacteria and efflux-deficient strains of Gram-negative bacteria1-2, 5. Lpm functions by inhibiting bacterial RNA polymerase (RNAP)6-8. Lpm exhibits no cross-resistance with the classic RNAP inhibitor rifampin (Rif)7, 9 and inhibits transcription initiation at an earlier step than Rif8-11, suggesting that the binding site and mechanism of Lpm differ from those of Rif. Efforts spanning a decade to obtain a crystal structure of RNAP in complex with Lpm have been unsuccessful. Here, we report a cryo-EM12-13 structure of Mycobacterium tuberculosis RNAP holoenzyme in complex with Lpm at 3.5 [A] resolution. The structure shows that Lpm binds at the base of the RNAP \"clamp,\" interacting with the RNAP switch region and the RNAP RNA exit channel. The binding site on RNAP for Lpm does not overlap the binding sites for other RNAP inhibitors, accounting for the absence of cross-resistance of Lpm with other RNAP inhibitors. The structure exhibits an open conformation of the RNAP clamp, with the RNAP clamp swung outward by ~17{degrees} relative to its position in catalytically competent RNAP-promoter transcription initiation complexes, suggesting that Lpm traps an open-clamp conformational state. Single-molecule fluorescence resonance energy transfer14 experiments confirm that Lpm traps an open-clamp conformational state and define effects of Lpm on clamp opening and closing dynamics. We propose that Lpm inhibits transcription initiation by trapping an open-clamp conformational state, thereby preventing simultaneous engagement of transcription initiation factor {sigma} regions 2 and 4 with promoter -10 and -35 elements. The results provide information essential to understanding the mode of action of Lpm, account for structure-activity relationships of known Lpm analogs, and suggest modifications to Lpm that could yield new, improved Lpm analogs.

molecular biology↗