Laminar flow cells for single-molecule studies of DNA-protein interactions

Microfluidic flow cells are used in single-molecule experiments, enabling measurements to be made with high spatial and temporal resolution. We discuss the fundamental processes affecting flow cell operation and describe the flow cells in use at present for studying the interaction of optically trapped or mechanically isolated, single DNA molecules with proteins. To assist the experimentalist in flow cell selection, we review the construction techniques and materials used to fabricate both single-and multiple-channel flow cells and the advantages of each design for different experiments

Single-molecule studies of fork dynamics in Escherichia coli DNA replication

We present single-molecule studies of the Escherichia coli replication machinery. We visualize individual E. coli DNA polymerase III (Pol III) holoenzymes engaging in primer extension and leading-strand synthesis. When coupled to the replicative helicase DnaB, Pol III mediates leading-strand synthesis with a processivity of 10.5 kilobases (kb), eight-fold higher than that by Pol III alone. Addition of the primase DnaG causes a three-fold reduction in the processivity of leading-strand synthesis, an effect dependent upon the DnaB-DnaG protein-protein interaction rather than primase activity. A single-molecule analysis of the replication kinetics with varying DnaG concentrations indicates that a cooperative binding of two or three DnaG monomers to DnaB halts synthesis. Modulation of DnaB helicase activity through the interaction with DnaG suggests a mechanism that prevents leading-strand synthesis from outpacing lagging-strand synthesis during slow primer synthesis on the lagging strand.

Physical virology

Viruses are nanosized, genome-filled protein containers with remarkable thermodynamic and mechanical properties. They form by spontaneous self-assembly inside the crowded, heterogeneous cytoplasm of infected cells. Self-assembly of viruses seems to obey the principles of thermodynamically reversible self-assembly but assembled shells ('capsids') strongly resist disassembly. Following assembly, some viral shells pass through a sequence of coordinated maturation steps that progressively strengthen the capsid. Nanoindentation measurements by atomic force microscopy enable tests of the strength of individual viral capsids. They show that concepts borrowed from macroscopic materials science are surprisingly relevant to viral shells. For example, viral shells exhibit 'materials fatigue- and the theory of thin-shell elasticity can account - in part - for atomic-force-microscopy-measured force-deformation curves. Viral shells have effective Young's moduli ranging from that of polyethylene to that of plexiglas. Some of them can withstand internal osmotic pressures that are tens of atmospheres. Comparisons with thin-shell theory also shed light on nonlinear irreversible processes such as plastic deformation and failure. Finally, atomic force microscopy experiments can quantify the mechanical effects of genome encapsidation and capsid protein mutations on viral shells, providing virological insight and suggesting new biotechnological applications. © 2010 Macmillan Publishers Limited. All rights reserved