DNA transcription by RNA polymerase II (RNApII) is arguably the process most central to regulation of gene expression in eukaryotic organisms. Regulated transcription requires the formation on DNA of molecular assemblies containing not only RNApII but also dozens of accessory proteins that play pivotal roles in the process. While we know about the structures of some of these assemblies in atomic detail, quantitative understanding of the dynamics and pathways by which the assemblies interconvert and progress through this fundamental gene expression pathway is largely lacking.
In this study we report single-molecule fluorescence microscopy studies of transcription in yeast nuclear extract, for the first time visualizing and measuring the dynamics of activator-dependent recruitment of RNApII and the central elongation factor Spt4/5 to transcription complexes. Grace Rosen (Jeff Gelles’ labortatory, Brandeis) , Inwha Baek (Steve, Buratowski’s lab, Harvard Medical School), and collaborators elucidated the kinetically significant steps in activated RNApII transcription initiation and show for the first time that Spt4/5 dynamics are tuned to the typical lifetimes of transcription elongation complexes. In addition to these substantive results, our work represents an important methodological advance. As the first application of the CoSMoS (co-localization single-molecule spectroscopy) technique to activated eukaryotic transcription, it demonstrates a general method for elucidating the correlated dynamic interactions of different components of the machinery with initiation and elongation transcription complexes. The approach is likely to find further use in studies of the mechanistic features of RNApII transcription.
Rosen, G.A., Baek, I., et al., Dynamics of RNA polymerase II and elongation factor Spt4/5 recruitment during activator-dependent transcription
The dynamic assembly, remodeling, and turnover of actin networks drives cellular processes
ranging from cell motility, endocytosis, and phagocytosis to cell division, cell and tissue
morphogenesis, and neuronal pathfinding. Here, we describe a new actin regulatory activity that changes understanding of how actin networks can be turned over. In a collaborative project with Bruce Goode’s lab, postdocs Shashank Shekhar and Greg Hoeprich used microfluidics-assisted total internal reflection fluorescence (TIRF) microscopy to show that mammalian twinfilin, an evolutionary conserved ADF/cofilin-homology protein, accelerates depolymerization at newly-assembled (ADP-Pi) but not older (ADP) actin filaments, even under assembly-promoting conditions (i.e., at G-actin concentrations above the critical concentration). Our data suggest that twinfilin molecules interact processively with the barbed end of the filament as it shrinks, blocking ATP-actin subunit addition while allowing ADP-Pi subunit dissociation. These novel activities of twinfilin reveal that cells have machinery that can bypass the normal filament aging process and induce the depolymerization of barbed ends as needed. These results may explain known genetic interactions between twinfilin and cofilin, and localization of twinfilin to the tips of filopodia and stereocilia, where actin filament barbed ends are clustered.
Shekhar S, et al., Twinfilin bypasses assembly conditions and actin filament aging to drive barbed end depolymerization.
Journal of Cell Biology 220, e202006022 (2021)
Congratulations to Dr. Shashank Shekhar, who has established his independent research lab in the Department of Physics at Emory University, where he will continue studying the biophysics and biochemistry of the actin cytoskeleton. We have been pleased to have Shashank at Brandeis over the past few years as a cross-departmental postdoc in the labs of Bruce Goode (Biology), Jeff Gelles (Biochemistry), and Jane Kondev (Physics).
DNA transcription is the most important nexus of gene regulation in all living organisms. In recent years, single-molecule experiments have given us a new window into the mechanisms of transcription and have revealed novel, previously unsuspected molecular behaviors and mechanisms. Ph.D. student Tim Harden used multi-color single-molecule fluorescence imaging to reveal a completely new transcription cycle for bacterial RNA polymerase. Harden and co-authors showed that an RNA polymerase molecule in vitro frequently (in >90% of transcription events) remains bound to DNA and may again initiate transcription after it has terminated the first round of transcription. Even more unexpectedly, this “secondary initiation” is not restricted to the same RNA. After the first round, the polymerase can scan thousands of basepairs along the DNA and can initiate at a different start site, frequently one that is oriented in the opposite direction and produces an antisense transcript.
To complement the single-molecule studies in vitro, the manuscript reports new analyses of whole-transcriptome cellular RNAs revealed by the Rend-seq method that measures transcript initiation and termination frequencies across the genome with single basepair resolution. These provide evidence that the new transcription cycle may be responsible for initiating antisense transcription at hundreds of genomic locations in the two widely divergent bacterial species examined. The work defines a new mechanism for the regulated production of antisense RNAs, many of which are now recognized as important agents of gene-specific regulation through control of transcription, mRNA decay, and translation. In addition, the new transcription cycle provides a mechanism through which transcription initiation can be controlled not just through feedback networks involving multiple genes, but also through production of multiple different primary transcripts consequent to a single RNA polymerase-to-DNA recruitment event.
Harden, T.T., et al. Alternative transcription cycle for bacterial RNA polymerase.
Nature Communications 11, 450 (2020).
From Science at Brandeis:
“All animal and plant cells contain a highly elaborate system of filamentous protein polymers called the actin cytoskeleton, a scaffold that can be rapidly transformed to alter a cell’s shape and function. A critical step in reconfiguring this scaffold is the rapid disassembly (or turnover) of the actin filaments. But how is this achieved? It has long been known that the protein Cofilin plays a central role in this process, but it has been unclear how Cofilin achieves this feat. Cofilin can sever actin filaments into smaller fragments to promote their disassembly, but whether it also catalyzes subunit dissociation from filament ends has remained uncertain and controversial. Until now, this problem has been difficult to address because of limitations in directly observing Cofilin’s biochemical effects at filament ends….” Dr. Shashank Shekhar, working together with Dr. Johnson Chung and “jointly mentored by Bruce Goode, Jeff Gelles and Jane Kondev, use[d] microfluidics-assisted single molecule TIRF imaging to tackle the problem.
The new study shows that Cofilin and one other protein (Srv2/CAP) intimately collaborate at one end of the actin filament to accelerate subunit dissociation by over 300-fold! These are the fastest rates of actin depolymerization ever observed. Further, these results establish a new paradigm in which a protein that decorates filament sides (Cofilin) works in concert with a protein that binds to filament ends (Srv2/CAP) to produce an activity that is orders of magnitude stronger than the that of either protein alone.”
Shekhar S. et al. Synergy between cyclase-associated protein and cofilin accelerates actin filament depolymerization by two orders of magnitude.
Nature Communications 10, 5319 (2019).
In this project, Kankowan Champasa from Stephen Bell’s lab at MIT collaborated with other researchers from the Bell and Gelles labs to study a key process that sets the stage for replication of chromosomal DNA. They explain “licensing of eukaryotic origins of replication requires DNA loading of two copies of the Mcm2-7 replicative helicase to form a head-to-head double-hexamer, ensuring activated helicases depart the origin bidirectionally.” The researchers identified a conserved motif in the Mcm4 helicase subunit essential for formation of productive replication complexes. Single-molecule fluorescence energy transfer experiments show that mutations in the motif still allow the two hexamers to come into contact, but they prevent the formation of the stable double-hexamers that perform the extensive DNA unwinding needed for replication.
A conserved Mcm4 motif is required for Mcm2-7 double-hexamer formation and origin DNA unwinding.
Champasa, K., Blank, C., Friedman, L.J., Gelles, J., and Bell, S.P.
eLife (2019) 8:e40576
Grp94 is a molecular chaperone that helps to fold and maintain the folded state of “client” proteins in the endoplasmic reticulum. Acceleration of client folding is driven by conformational changes in Grp94. However, the sequence of conformational changes and how these changes are coupled to the cycle of ATP hydrolysis is not well understood. Prof. Timothy Street and his lab members Bin Huang and Ming Sun, in collaboration with Larry Friedman, did single-molecule fluorescence resonance energy transfer (FRET) experiments to directly observe conformational cycling in individual Grp94 molecules. Their studies show that ATP hydrolysis can drive repeated cycling between alternative “closed” states of Grp94, suggesting a way that enzyme might propagate structural changes to client molecules.
Conformational Cycling within the Closed State of Grp94, an Hsp90-Family Chaperone
Huang, B., Friedman, L.J., Gelles, J., Sun, M., and Street, T.O.
Journal of Molecular Biology 431, 3312-3323 (2019).
Congratulations to Grace Rosen on completing her Ph.D. defense! Grace will be working post-graduation as a scientist at the Boston VA Medical Center.
RNA polymerases contain a conserved “secondary channel” in which proteins that regulate transcription can bind. In Escherichia coli bacteria, the secondary channel factors (SCFs) GreB and DksA both repress initiation of ribosomal RNA synthesis, but SCF loading and repression mechanisms are unclear. Sarah Stumper and her collaborators used fluorescence correlation spectroscopy and multi-wavelength single-molecule fluorescence colocalization microscopy to show that the SCFs likely repress transcription through an interesting “delayed inhibition” mechanism in which the proteins arrive at DNA already complexed to RNA polymerase and block at a later stage of transcription initiation. The work explains factors that control the relative contributions of the two proteins to regulation and suggests a mechanism by which repression is restricted to ribosomal RNA and other promoters that form short-duration complexes with RNA polymerase.
Delayed inhibition mechanism for secondary channel factor regulation of ribosomal RNA transcription.
Stumper, S.K., Ravi, H., Friedman, L.J., Mooney, R.A., Corrêa, I.R., Gershenson, A., Landick, R., and Gelles, J.
eLife (2019) 8:e40576