Duplex DNA and BLM regulate gate opening by the human TopoIIIα-RMI1-RMI2 complex.

Topoisomerase IIIα is a type 1A topoisomerase that forms a complex with RMI1 and RMI2 called TRR in human cells. TRR plays an essential role in resolving DNA replication and recombination intermediates, often alongside the helicase BLM. While the TRR catalytic cycle is known to involve a protein-mediated single-stranded (ss)DNA gate, the detailed mechanism is not fully understood. Here, we probe the catalytic steps of TRR using optical tweezers and fluorescence microscopy. We demonstrate that TRR forms an open gate in ssDNA of 8.5 ± 3.8 nm, and directly visualize binding of a second ssDNA or double-stranded (ds)DNA molecule to the open TRR-ssDNA gate, followed by catenation in each case. Strikingly, dsDNA binding increases the gate size (by ~16%), while BLM alters the mechanical flexibility of the gate. These findings reveal an unexpected plasticity of the TRR-ssDNA gate size and suggest that TRR-mediated transfer of dsDNA may be more relevant in vivo than previously believed.

Virus self-assembly proceeds through contact-rich energy minima.

Self-assembly of supramolecular complexes such as viral capsids occurs prominently in nature. Nonetheless, the mechanisms underlying these processes remain poorly understood. Here, we uncover the assembly pathway of hepatitis B virus (HBV), applying fluorescence optical tweezers and high-speed atomic force microscopy. This allows tracking the assembly process in real time with single-molecule resolution. Our results identify a specific, contact-rich pentameric arrangement of HBV capsid proteins as a key on-path assembly intermediate and reveal the energy balance of the self-assembly process. Real-time nucleic acid packaging experiments show that a free energy change of ~1.4 kBT per condensed nucleotide is used to drive protein oligomerization. The finding that HBV assembly occurs via contact-rich energy minima has implications for our understanding of the assembly of HBV and other viruses and also for the development of new antiviral strategies and the rational design of self-assembling nanomaterials.

Photostability of Fullerene and Non-Fullerene Polymer Solar Cells: The Role of the Acceptor.

Recently, the advent of non-fullerene acceptors (NFAs) made it possible for organic solar cells (OSCs) to break the 10% efficiency barrier hardly attained by fullerene acceptors (FAs). In the past five years alone, more than hundreds of NFAs with applications in organic photovoltaics (OPVs) have been synthesized, enabling a notable current record efficiency of above 15%. Hence, there is a shift in interest toward the use of NFAs in OPVs. However, there has been little work on the stability of these new materials in devices. More importantly, there is very little comparative work on the photostability of FA versus NFA solar cells to ascertain the pros and cons of the two systems. Here, we show the photostability of solar cells based on two workhorse acceptors, in both conventional and inverted structures, namely, ITIC (as NFA) and [70]PCBM (as FA), blended with either PBDB-T or PTB7-Th polymer. We found that, irrespective of the polymer, the cell structure, or the initial efficiency, the [70]PCBM devices are more photostable than the ITIC ones. This observation, however, opposes the assumption that NFA solar cells are more photochemically stable. These findings suggest that complementary absorption should not take precedence in the design rules for the synthesis of new molecules and there is still work left to be done to achieve stable and efficient OSCs.