Nanopore technologies are being developed for fast and direct sequencing of single DNA molecules through detection of ionic current modulations as DNA passes through a pore's constriction 1,2 . Here we demonstrate the ability to resolve changes in current that correspond to a known DNA sequence by combining the high sensitivity of a mutated form of the protein pore Mycobacterium smegmatis porin A (MspA) 3 with phi29 DNA polymerase (DNAP) 4 , which controls the rate of DNA translocation through the pore. As phi29 DNAP synthesizes DNA and functions like a motor to pull a single-stranded template through MspA, we observe well-resolved and reproducible ionic current levels with median durations of ~28 ms and ionic current differences of up to 40 pA. Using six different DNA sequences with readable regions 42-53 nucleotides long, we record current traces that map to the known DNA sequences. With singlenucleotide resolution and DNA translocation control, this system integrates solutions to two longstanding hurdles to nanopore sequencing 2 .In nanopore DNA sequencing, a pore inserted into a membrane permits the flow of ionic current when a voltage is applied across the membrane. As a strand of DNA passes through the pore, it causes changes in current that can be related to the sequence of the DNA. Such a strategy offers the promise of rapidly sequencing long single molecules of DNA 2 , amplification-free sample preparation 1 and direct detection of epigenetic modifications such as base methylation 3,5 . Recent progress toward nanopore sequencing has included determining the resolution and characterizing the recognition sites of biological nanopores MspA 3,6 and α-hemolysin 7-9 as well as developing a method to slow DNA translocation though a nanopore 4,6,10 . However, to our knowledge no system has yet been reported that can read nucleotide-specific current levels as an unmodified strand of DNA passes through a nanopore. In this work, we read DNA by detecting current levels associated with singlenucleotide movement of the strand through MspA. Base-calling algorithms will need to be developed to translate these ionic current reads into a DNA sequence.

Abstract. We briefly summarize motivations for testing the weak equivalence principle and then review recent torsion-balance results that compare the differential accelerations of beryllium-aluminum and beryllium-titanium test body pairs with precisions at the part in 10 13 level. We discuss some implications of these results for the gravitational properties of antimatter and dark matter, and speculate about the prospects for further improvements in experimental sensitivity.

Motivated by higher-dimensional theories that predict new effects, we tested the gravitational 1/r(2) law at separations ranging down to 218 microm using a 10-fold symmetric torsion pendulum and a rotating 10-fold symmetric attractor. We improved previous short-range constraints by up to a factor of 1000 and find no deviations from Newtonian physics.

Nanopores hold great promise as single-molecule analytical devices and biophysical model systems because the ionic current blockades they produce contain information about the identity, concentration, structure, and dynamics of target molecules. The porin MspA of Mycobacterium smegmatis has remarkable stability against environmental stresses and can be rationally modified based on its crystal structure. Further, MspA has a short and narrow channel constriction that is promising for DNA sequencing because it may enable improved characterization of short segments of a ssDNA molecule that is threaded through the pore. By eliminating the negative charge in the channel constriction, we designed and constructed an MspA mutant capable of electronically detecting and characterizing single molecules of ssDNA as they are electrophoretically driven through the pore. A second mutant with additional exchanges of negatively-charged residues for positively-charged residues in the vestibule region exhibited a factor of Ϸ20 higher interaction rates, required only half as much voltage to observe interaction, and allowed ssDNA to reside in the vestibule Ϸ100 times longer than the first mutant. Our results introduce MspA as a nanopore for nucleic acid analysis and highlight its potential as an engineerable platform for single-molecule detection and characterization applications.DNA sequencing ͉ protein engineering ͉ bio-nanotechnology