Sequence-dependent mechanochemical coupling of helicase translocation and unwinding at single-nucleotide resolution.

We used single-molecule picometer-resolution nanopore tweezers (SPRNT) to resolve the millisecond single-nucleotide steps of superfamily 1 helicase PcrA as it translocates on, or unwinds, several kilobase-long DNA molecules. We recorded more than two million enzyme steps under various assisting and opposing forces in diverse adenosine tri- and diphosphate conditions to comprehensively explore the mechanochemistry of PcrA motion. Forces applied in SPRNT mimic forces and physical barriers PcrA experiences in vivo, such as when the helicase encounters bound proteins or duplex DNA. We show how PcrA's kinetics change with such stimuli. SPRNT allows for direct association of the underlying DNA sequence with observed enzyme kinetics. Our data reveal that the underlying DNA sequence passing through the helicase strongly influences the kinetics during translocation and unwinding. Surprisingly, unwinding kinetics are not solely dominated by the base pairs being unwound. Instead, the sequence of the single-stranded DNA on which the PcrA walks determines much of the kinetics of unwinding.

Determining the effects of DNA sequence on Hel308 helicase translocation along single-stranded DNA using nanopore tweezers.

Motor enzymes that process nucleic-acid substrates play vital roles in all aspects of genome replication, expression, and repair. The DNA and RNA nucleobases are known to affect the kinetics of these systems in biologically meaningful ways. Recently, it was shown that DNA bases control the translocation speed of helicases on single-stranded DNA, however the cause of these effects remains unclear. We use single-molecule picometer-resolution nanopore tweezers (SPRNT) to measure the kinetics of translocation along single-stranded DNA by the helicase Hel308 from Thermococcus gammatolerans. SPRNT can measure enzyme steps with subangstrom resolution on millisecond timescales while simultaneously measuring the absolute position of the enzyme along the DNA substrate. Previous experiments with SPRNT revealed the presence of two distinct substates within the Hel308 ATP hydrolysis cycle, one [ATP]-dependent and the other [ATP]-independent. Here, we analyze in-depth the apparent sequence dependent behavior of the [ATP]-independent step. We find that DNA bases at two sites within Hel308 control sequence-specific kinetics of the [ATP]-independent step. We suggest mechanisms for the observed sequence-specific translocation kinetics. Similar SPRNT measurements and methods can be applied to other nucleic-acid-processing motor enzymes.