High-throughput platform for real-time monitoring of biological processes by multicolor single-molecule fluorescence.

Zero-mode waveguides provide a powerful technology for studying single-molecule real-time dynamics of biological systems at physiological ligand concentrations. We customized a commercial zero-mode waveguide-based DNA sequencer for use as a versatile instrument for single-molecule fluorescence detection and showed that the system provides long fluorophore lifetimes with good signal to noise and low spectral cross-talk. We then used a ribosomal translation assay to show real-time fluidic delivery during data acquisition, showing it is possible to follow the conformation and composition of thousands of single biomolecules simultaneously through four spectral channels. This instrument allows high-throughput multiplexed dynamics of single-molecule biological processes over long timescales. The instrumentation presented here has broad applications to single-molecule studies of biological systems and is easily accessible to the biophysical community.

Pulling on the nascent RNA during transcription does not alter kinetics of elongation or ubiquitous pausing.

Transcriptional elongation and termination by RNA polymerase (RNAP) are controlled by interactions among the nascent RNA, DNA, and RNAP that comprise the ternary transcription elongation complex (TEC). To probe the effects of cotranscriptionally folded RNA hairpins on elongation as well as the stability of the TEC, we developed a single-molecule assay to monitor RNA elongation by Escherichia coli RNAP molecules while applying controlled loads to the nascent RNA that favor forward translocation. Remarkably, forces up to 30 pN, twice those required to disrupt RNA secondary structure, did not significantly affect enzyme processivity, transcription elongation rates, pause frequencies, or pause lifetimes. These results indicate that ubiquitous transcriptional pausing is not a consequence of the formation of hairpins in the nascent RNA. The ability of the TEC to sustain large loads on the transcript reflects a tight binding of RNA within the TEC and has important implications for models of transcriptional termination.

Forward and reverse motion of single RecBCD molecules on DNA.

RecBCD is a processive, DNA-based motor enzyme with both helicase and nuclease activities. We used high-resolution optical trapping to study individual RecBCD molecules moving against applied forces up to 8 pN. Fine-scale motion was smooth down to a detection limit of 2 nm, implying a unitary step size below six basepairs (bp). Episodes of constant-velocity motion over hundreds to thousands of basepairs were punctuated by abrupt switches to a different speed or by spontaneous pauses of mean length 3 s. RecBCD occasionally reversed direction, sliding backward along DNA. Backsliding could be halted by reducing the force, after which forward motion sometimes resumed, often after a delay. Elasticity measurements showed that the DNA substrate was partially denatured during backsliding events, but reannealed concomitant with the resumption of forward movement. Our observations show that RecBCD-DNA complexes can exist in multiple, functionally distinct states that persist for many catalytic turnovers: such states may help tune enzyme activity for various biological functions.

Sequence-dependent pausing of single lambda exonuclease molecules.

Lambda exonuclease processively degrades one strand of duplex DNA, moving 5'-to-3' in an ATP-independent fashion. When examined at the single-molecule level, the speeds of digestion were nearly constant at 4 nanometers per second (12 nucleotides per second), interspersed with pauses of variable duration. Long pauses, occurring at stereotypical locations, were strand-specific and sequence-dependent. Pause duration and probability varied widely. The strongest pause, GGCGAT TCT, was identified by gel electrophoresis. Correlating single-molecule dwell positions with sequence independently identified the motif GGCGA. This sequence is found in the left lambda cohesive end, where exonuclease inhibition may contribute to the reduced recombination efficiency at that end.