Motor enzymes that interact with DNA are essential for replicative biological processes. In nanopore sequencing, a motor enzyme controls the motion of a nucleic acid through a protein nanopore, and sequence-dependent blockages of an ion current flowing through the nanopore are used to decode the DNA sequence. The kinetics of these enzymes are sequence-dependent and can serve as an additional source of information during sequencing. Here, we use Mutual Information (MI) to quantify the sequence-dependent kinetics of a Hel308 helicase during nanopore sequencing. We use MI to identify sites in Hel308 that are responsible for sequence-dependent kinetics and develop "k-mer" models of Hel308 kinetics that map kinetics to DNA sequence. We estimate that enzyme kinetics can improve nanopore sequencing accuracy by ~5-fold at high sequencing depth. We mutate Hel308 to identify amino acids involved in DNA translocation and suggest pathways for engineering molecular motors with enhanced responsiveness to DNA sequence.

"ALternative Isoinformational ENgineered" (ALIEN) DNA is a biomimetic polymer composed of four entirely anthropogenic nucleotides. These alternative nucleosides form base pairs orthogonal to canonical bases and fold into the familiar B-form DNA double-helix, endowing ALIEN DNA with valuable biotechnological applications. The ability to sequence ALIEN DNA is essential for its continued development. However traditional sequencing approaches rely on chemical recognition of ACGT-DNA and cannot be easily adapted to ALIEN DNA. Here we demonstrate de novo nanopore sequencing of DNA comprised entirely of the four anthropogenic DNA bases. We show direct, label-free, single-molecule sequencing of such nucleic acids without the requirements of fluorescent labels, transliteration, amplification, or enzymatic synthesis. This paves the way for routine, accessible, and high-accuracy sequencing of DNA beyond A, C, G, and T.

β-Barrel nanopores are involved in crucial biological processes, from ATP export in mitochondria to bacterial resistance, and represent a promising platform for emerging sequencing technologies. However, in contrast to ion channels, the understanding of the fundamental principles governing ion transport through these nanopores remains largely unexplored. Here we integrate experimental, numerical and theoretical approaches to elucidate ion transport mechanisms in β-barrel nanopores. We identify and characterize two distinct nonlinear phenomena: open-pore rectification and gating. Through extensive mutation analysis of aerolysin nanopores, we demonstrate that open-pore rectification is caused by ionic accumulation driven by the distribution of lumen charges. In addition, we provide converging evidence suggesting that gating is controlled by electric fields dissociating counterions from lumen charges, promoting local structural deformations. Our findings establish a rigorous framework for characterizing and understanding ion transport processes in protein-based nanopores, enabling the design of adaptable nanofluidic biotechnologies. We illustrate this by optimizing an aerolysin mutant for computing applications.

Peptide phytohormones are decorated with post-translational modifications (PTMs) that are crucial for receptor recognition. Tyrosine sulfation on these hormones is essential for plant growth and development1. Measuring the occurrence and position of sulfotyrosine is, however, compromised by major technical challenges during isolation and detection2. We recently introduced a nanopore peptide sequencing method that sensitively detects PTMs at the single-molecule level3. By translocating PTM variants of the plant pentapeptide hormone phytosulfokine (PSK) through a nanopore, we here demonstrate accurate identification of sulfation and phosphorylation on the two tyrosine residues of PSK. Sulfation can be clearly detected and distinguished (>90%) from phosphorylation on the same residue. Moreover, the presence or absence of PTMs on the two close-by tyrosine residues can be accurately determined (>96% accuracy). Our findings demonstrate the extraordinary sensitivity of nanopore protein measurements, providing a new tool for identifying sulfation on peptide phytohormones and promising wider applications to identify protein PTMs.

Peptide hormones are decorated with post-translational modifications (PTMs) that are crucial for receptor recognition. Tyrosine sulfation on plant peptide hormones is, for example, essential for plant growth and development. Measuring the occurrence and position of sulfotyrosine is, however, compromised by major technical challenges during isolation and detection. Nanopores can sensitively detect protein PTMs at the single-molecule level. By translocating PTM variants of the plant pentapeptide hormone phytosulfokine (PSK) through a nanopore, we here demonstrate the accurate identification of sulfation and phosphorylation on the two tyrosine residues of PSK. Sulfation can be clearly detected and distinguished (>90%) from phosphorylation on the same residue. Moreover, the presence or absence of PTMs on the two close-by tyrosine residues can be accurately determined (>96% accuracy). Our findings demonstrate the extraordinary sensitivity of nanopore protein measurements, providing a powerful tool for identifying position-specific sulfation on peptide hormones and promising wider applications to identify protein PTMs.

Nanopores are increasingly powerful tools for single molecule sensing, in particular, for sequencing DNA, RNA and peptides. This success has spurred efforts to sequence non-canonical nucleic acid bases and amino acids. While canonical DNA and RNA bases have pKas far from neutral, certain non-canonical bases, natural RNA modifications, and amino acids are known to have pKas near neutral pHs at which nanopore sequencing is typically performed. Previous reports have suggested that the nanopore signal may be sensitive to the protonation state of an individual moiety. We sequenced ion currents with the MspA nanopore using a single stranded DNA containing a single non-canonical DNA base (Z) at various pH conditions. The Z-base has a near-neutral pKa ∼ 7.8. We find that the measured ion current is remarkably sensitive to the protonation state of the Z-base. We demonstrate how nanopores can be used to localize and determine the pKa of individual moieties along a polymer. More broadly, these experiments provide a path to mapping different protonation sites along polymers and give insight in how to optimize sequencing of polymers that contain moieties with near-neutral pKas.

RNA polymerases (RNAPs) carry out the first step in the central dogma of molecular biology by transcribing DNA into RNA. Despite their importance, much about how RNAPs work remains unclear, in part because the small (3.4 Angstrom) and fast (~40 ms/nt) steps during transcription were difficult to resolve. Here, we used high-resolution nanopore tweezers to observe the motion of single Escherichia coli RNAP molecules as it transcribes DNA ~1,000 times improved temporal resolution, resolving single-nucleotide and fractional-nucleotide steps of individual RNAPs at saturating nucleoside triphosphate concentrations. We analyzed RNAP during processive transcription elongation and sequence-dependent pausing at the yrbL elemental pause sequence. Each time RNAP encounters the yrbL elemental pause sequence, it rapidly interconverts between five translocational states, residing predominantly in a half-translocated state. The kinetics and force-dependence of this half-translocated state indicate it is a functional intermediate between pre- and post-translocated states. Using structural and kinetics data, we show that, in the half-translocated and post-translocated states, sequence-specific protein–DNA interaction occurs between RNAP and a guanine base at the downstream end of the transcription bubble (core recognition element). Kinetic data show that this interaction stabilizes the half-translocated and post-translocated states relative to the pre-translocated state. We develop a kinetic model for RNAP at the yrbL pause and discuss this in the context of key structural features.

Current methods to detect post-translational modifications of proteins, such as phosphate groups, cannot measure single molecules or differentiate between closely spaced phosphorylation sites. We detect post-translational modifications at the single-molecule level on immunopeptide sequences with cancer-associated phosphate variants by controllably drawing the peptide through the sensing region of a nanopore. We discriminate peptide sequences with one or two closely spaced phosphates with 95% accuracy for individual reads of single molecules.

Chemists have now synthesized new kinds of DNA that add nucleotides to the four standard nucleotides (guanine, adenine, cytosine, and thymine) found in standard Terran DNA. Such "artificially expanded genetic information systems" are today used in molecular diagnostics; to support directed evolution to create medically useful receptors, ligands, and catalysts; and to explore issues related to the early evolution of life. Further applications are limited by the inability to directly sequence DNA containing nonstandard nucleotides. Nanopore sequencing is well-suited for this purpose, as it does not require enzymatic synthesis, amplification, or nucleotide modification. Here, we take the first steps to realize nanopore sequencing of an 8-letter "hachimoji" expanded DNA alphabet by assessing its nanopore signal range using the MspA (Mycobacterium smegmatis porin A) nanopore. We find that hachimoji DNA exhibits a broader signal range in nanopore sequencing than standard DNA alone and that hachimoji single-base substitutions are distinguishable with high confidence. Because nanopore sequencing relies on a molecular motor to control the motion of DNA, we then assessed the compatibility of the Hel308 motor enzyme with nonstandard nucleotides by tracking the translocation of single Hel308 molecules along hachimoji DNA, monitoring the enzyme kinetics and premature enzyme dissociation from the DNA. We find that Hel308 is compatible with hachimoji DNA but dissociates more frequently when walking over C-glycoside nucleosides, compared to N-glycosides. C-glycocide nucleosides passing a particular site within Hel308 induce a higher likelihood of dissociation. This highlights the need to optimize nanopore sequencing motors to handle different glycosidic bonds. It may also inform designs of future alternative DNA systems that can be sequenced with existing motors and pores.

The genome of SARS-CoV-2 encodes for a helicase called nsp13 that is essential for viral replication and highly conserved across related viruses, making it an attractive antiviral target. Here we use nanopore tweezers, a high-resolution single-molecule technique, to gain detailed insight into how nsp13 turns ATP-hydrolysis into directed motion along nucleic acid strands. We measured nsp13 both as it translocates along single-stranded DNA or unwinds short DNA duplexes. Our data confirm that nsp13 uses the inchworm mechanism to move along the DNA in single-nucleotide steps, translocating at ~1000 nt/s or unwinding at ~100 bp/s. Nanopore tweezers' high spatio-temporal resolution enables observation of the fundamental physical steps taken by nsp13 even as it translocates at speeds in excess of 1000 nucleotides per second enabling detailed kinetic analysis of nsp13 motion. As a proof-of-principle for inhibition studies, we observed nsp13's motion in the presence of the ATPase inhibitor ATPγS. Our data reveals that ATPγS interferes with nsp13's action by affecting several different kinetic processes. The dominant mechanism of inhibition differs depending on the application of assisting force. These advances demonstrate that nanopore tweezers are a powerful method for studying viral helicase mechanism and inhibition.

A proteomics tool capable of identifying single proteins would be important for cell biology research and applications. Here, we demonstrate a nanopore-based single-molecule peptide reader sensitive to single–amino acid substitutions within individual peptides. A DNA-peptide conjugate was pulled through the biological nanopore MspA by the DNA helicase Hel308. Reading the ion current signal through the nanopore enabled discrimination of single–amino acid substitutions in single reads. Molecular dynamics simulations showed these signals to result from size exclusion and pore binding. We also demonstrate the capability to “rewind” peptide reads, obtaining numerous independent reads of the same molecule, yielding an error rate of <10 −6 in single amino acid variant identification. These proof-of-concept experiments constitute a promising basis for the development of a single-molecule protein fingerprinting and analysis technology.

As identifying proteins is of paramount importance for cell biology and applications, it is of interest to develop a protein sequencer with the ultimate sensitivity of decoding individual proteins. Here, we demonstrate a nanopore-based single-molecule sequencing approach capable of reliably detecting single amino-acid substitutions within individual peptides. A peptide is linked to a DNA molecule that is pulled through the biological nanopore MspA by a DNA helicase in single amino-acid steps. The peptide sequence yields clear stepping ion current signals which allows to discriminate single-amino-acid substitutions in single reads. Molecular dynamics simulations show these signals to result from size exclusion and pore binding. Notably, we demonstrate the capability to ‘rewind’ peptide reads, obtaining indefinitely many independent reads of the same individual molecule, yielding virtually 100% read accuracy in variant identification, with an error rate less than 10 −6 . These proof-of-concept experiments constitute a promising basis for developing a single-molecule protein sequencer. One-sentence summary This paper presents proof-of-concept experiments and simulations of a nanopore-based approach to sequencing individual proteins.

Single-molecule Picometer Resolution Nanopore Tweezers (SPRNT) is a technique that enables observation of single enzyme movement along nucleic acids under an applied force. SPRNT provides measurements of enzyme position along a nucleic acid substrate with sub-Angstrom spatial and millisecond temporal resolution, while simultaneously providing the DNA sequence within the enzyme. We use SPRNT to monitor many E. coli RNA Polymerase (RNAP) core complexes during transcription elongation and pausing with an assisting force. We determine that during elongation at low [NTP], RNAP primarily stalls in a post-translocated state, with brief deviations forward to a hyper-translocated state and backwards to a pre-translocated state. The rates and frequencies of these transitions vary significantly with DNA sequence and the magnitude of assisting force. During transcription pausing at an elemental pause sequence, we observe transitions between five distinct enzyme states (backtracked, pre-, half-, post-, and hyper translocated), including a half-translocated state between pre and post. We develop a model for RNAP pausing and elongation by varying the applied force and monitoring RNAP mutants with SPRNT.

The next wave of research in bioinformatics and fundamental biology will revolve around sequencing the enormous and dynamic variations in the proteome. A method for sequencing proteins and detecting PTMs at the single-molecule level would be revolutionary for proteomics research. We report proof-of-concept and initial results for a new single-molecule protein sequencing technique. We use a typical enzyme-controlled biological nanopore sequencing setup, with the addition of a peptide fragment covalently linked to the end of the DNA strand. The nanopore sequencer first reads the DNA, then the peptide fragment as the DNA-peptide conjugate is pulled up through the nanopore. The readout is then used to decode the peptide sequence, map substitutions and post-translation modifications, or identify the peptide from a library of proteins. Similarly to nanopore DNA sequencing, this technique is in principle capable of single-molecule measurements, high throughput, low experimental overhead and minimal sample preparation, and sensitivity to changes in individual monomers.

Single-molecule picometer resolution nanopore tweezers (SPRNT) is a single-molecule technique that enables observation of enzyme movement along nucleic acids under an applied force. SPRNT measures enzyme position with sub-Angstrom spatial and millisecond temporal resolution, while simultaneously providing DNA sequence within the enzyme. We use SPRNT to monitor many E. coli RNA Polymerase (RNAP) core complexes during transcription elongation and pausing with an assisting force. We determine that during elongation at low [NTP], RNAP primarily stalls in a post-translocated state, with brief deviations forward to a hyper-translocated state and backwards to a pre-translocated state. The rates and frequencies of these transitions vary with DNA sequence. During transcription pausing at a pre-determined pause sequence, we observe RNAP entering a partially-translocated state, with brief deviations to backtracked, pre-, post- and hyper-translocated states. We develop a model for RNAP pausing by varying the applied force and monitoring RNAP mutants with SPRNT.

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.

Nanopore DNA sequencing is limited by low base-calling accuracy. Improved base-calling accuracy has so far relied on specialized base-calling algorithms, different nanopores and motor enzymes, or biochemical methods to re-read DNA molecules. Two primary error modes hamper sequencing accuracy: enzyme mis-steps and sequences with indistinguishable signals. We vary the driving voltage from 100 to 200 mV, with a frequency of 200 Hz, across a Mycobacterium smegmatis porin A (MspA) nanopore, thus changing how the DNA strand moves through the nanopore. A DNA helicase moves the DNA through the nanopore in discrete steps, and the variable voltage moves the DNA continuously between these steps. The electronic signal produced with variable voltage is used to overcome the primary error modes in base calling. We found that single-passage de novo base-calling accuracy of 62.7 ± 0.5% with a constant driving voltage improves to 79.3 ± 0.3% with a variable driving voltage. The variable-voltage sequencing mode is complementary to other methods to boost the accuracy of nanopore sequencing and could be incorporated into any enzyme-actuated nanopore sequencing device.

We applied a new single-molecule technique named SPRNT (Single-molecule Picometer Resolution Nanopore Tweezers) to analyze the translocation of single E. coli RNA polymerase (RNAP) core complexes during transcription. Inspired by nanopore DNA sequencing with biological porin MspA, SPRNT enables observation of motor protein movement along nucleic acids with sub-nucleotide spatial resolution, sub-millisecond temporal resolution, and the ability to apply assisting and opposing forces. In addition, the sequence in the RNAP complex is provided by nanopore sequencing, enabling the study of sequence-dependent translocation behaviors. Using SPRNT, we monitored RNAP dynamics at the sequence-specific pause element yrbL. By tracking hundreds of RNA polymerase molecules while varying applied force and [NTPs] over three orders of magnitude, we reveal, in unprecedented detail, the various enzyme states and complex kinetics during this pause element.