Single DNA molecules labeled with nanoparticles can be detected by blockades of ionic current as they are translocated through a nanopipette tip formed by a pulled glass capillary. The nanopipette detection technique can provide not only tools for detection and identification of single DNA and protein molecules but also deeper insight and understanding of stochastic interactions of various biomolecules with their environment.

We have fabricated a series of highly sensitive spin valve sensors on a micron scale that successfully detected the presence of a single superparamagnetic bead (Dynabeads M-280, 2.8 μm in diameter), and thus showed suitability for identifying biomolecules labeled by such magnetic beads. By polarizing the magnetic microbead on a spin valve sensor with a dc magnetic field and modulating its magnetization with an orthogonal ac magnetic field, we observed a magnetoresistance (MR) signal reduction caused by the magnetic dipole field from the bead that partially cancelled the applied fields to the spin valve. A lock-in technique was used to measure a voltage signal due to the MR reduction. A signal of 1.2 mV rms or 5.2 mΩ of resistance reduction was obtained from a 3 μm wide sensor and a signal of 3.8 mV rms or 11.9 mΩ from a 2.5 μm wide sensor. Micromagnetic simulations were also performed for the spin valve sensors with a single bead and gave results consistent with experiments. Further experiments and simulations suggested that these sensors or their variations can detect 1–10 Co nanoparticles with a diameter of ∼11 nm, and are suitable for DNA fragment detection.

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Spore formation in Bacillus subtilis involves a switch in the site of cell division from the midcell to a polar position. Both medial division and polar division are mediated in part by the actin-like, cytokinetic protein FtsA. We report the isolation of an FtsA mutant (FtsA D265G ) that is defective in sporulation but is apparently unimpaired in vegetative growth. Sporulating cells of the mutant reach the stage of asymmetric division but are partially blocked in the subsequent morphological process of engulfment. As judged by fluorescence microscopy and electron microscopy, the FtsA D265G mutant produces normal-looking medial septa but immature (abnormally thin) polar septa. The mutant was unimpaired in transcription under the control of Spo0A, the master regulator for entry into sporulation, but was defective in transcription under the control of F , a regulatory protein whose activation is known to depend on polar division. An amino acid substitution at a residue (Y264) adjacent to D265 also caused a defect in sporulation. D265 and Y264 are conserved among endospore-forming bacteria, raising the possibility that these residues are involved in a sporulation-specific protein interaction that facilitates maturation of the sporulation septum and the activation of F .Most bacteria divide by binary fission, forming a septum at the midpoint of the long axis of the cell to produce identicalsized progeny. The gram-positive soil bacterium Bacillus subtilis exhibits an additional mode of division when it enters the pathway to sporulate. Under such conditions, it undergoes a process of asymmetric division in which a septum is formed at a polar rather than a medial position. The polar septum divides the cell asymmetrically into a forespore (the smaller cell) and a mother cell. The forespore and the mother cell each receive a chromosome but exhibit dissimilar programs of gene expression (reviewed in reference 37). Gene expression in the forespore is governed by the transcription factor F , whose activation depends on the formation of the polar septum, whereas gene expression in the mother cell depends on the transcription factor E . Following asymmetric division, the forespore is engulfed by the mother cell in a phagocyte-like process that results in the forespore becoming pinched off as a free protoplast within the mother cell.The switch from medial division during growth to asymmetric division during sporulation involves a change in the subcellular localization of the cytokinetic protein FtsZ. This tubulinlike GTPase assembles into a ring-like structure, called the Z-ring, at the future site of septation (7, 49). During vegetative growth, the Z-ring assembles at the middle of the cell, fixing the position of the division site. The Z-ring is disassembled during cytokinesis and is largely absent from the completed septum. During sporulation, the Z-ring switches to a bipolar pattern of localization, forming a ring near each pole of the cell (29). Recent evidence indicates that the switch from medial to bipolar Z-rings occur...

The organization of the actin cytoskeleton plays an integral role in cell morphogenesis of all eukaryotes. We have isolated a temperature-sensitive mutant in Schizosaccharomyces pombe, wat1-1, in which acting patches are delocalized, resulting in an elliptically shaped cell phenotype. Molecular cloning and DNA sequencing of wat1+ showed that the gene encodes a 314 residue protein containing WD-40 repeats. Cells lacking wat1+ are slow growing but viable at 25 degrees C and temperature-sensitive for growth above 33 degrees C. At restrictive temperature, wat1-d strains are phenotypically indistinguishable from wat1-1. When combined with a deletion for the wat1+ gene, cdc mutants failed to elongate at restrictive temperature and exhibited alterations in actin patch localization. This analysis suggests that wat1+ is required directly or indirectly for polarized cell growth in S. pombe. Wat1p and a functional, epitope-tagged, version of Wat1p can be overproduced without inducing alterations in cell morphology.