The molecular mechanisms that regulate coordinated and colinear activation of Hox gene expression in space and time remain poorly understood. Here we demonstrate that Plzf regulates the spatial expression of the AbdB HoxD gene complex by binding to regulatory elements required for restricted Hox gene expression and can recruit histone deacetylases to these sites. We show by scanning forced microscopy that Plzf, via homodimerization, can form DNA loops and bridge distant Plzf binding sites located within HoxD gene regulatory elements. Furthermore, we demonstrate that Plzf physically interacts with Polycomb proteins on DNA. We propose a model by which the balance between activating morphogenic signals and transcriptional repressors such as Plzf establishes proper Hox gene expression boundaries in the limb bud.

The sequence-dependent curvature is generally recognized as an important and biologically relevant property of DNA because it is involved in the formation and stability of association complexes with proteins. When a DNA tract, intrinsically curved for the periodical recurrence on the same strand of A-tracts phased with the B-DNA periodicity, is deposited on a flat surface, it exposes to that surface either a T-or an A-rich face. The surface of a freshly cleaved mica crystal recognizes those two faces and preferentially interacts with the former one. Statistical analysis of scanning force microscopy (SFM) images provides evidence of this recognition between an inorganic crystal surface and nanoscale structures of double-stranded DNA. This finding could open the way toward the use of the sequence-dependent adhesion to specific crystal faces for nanotechnological purposes.I t is widely accepted that the local, sequence-dependent curvature and dynamics of the double-stranded DNA chain segments play a crucial and active role in DNA packaging, transcription, replication, recombination, and repair processes, and in nucleosome stability and positioning (1). Thus, a quantitative knowledge of the curvature and the flexibility of doublestranded DNA has become important to understand the determinants of DNA-protein recognition. We have recently described how SFM makes it possible to map sequencedependent curvature and flexibility along the DNA chain (2, 3). In these previous investigations, indications of preferential adsorption involving curved DNA tracts on the surface of mica were obtained. To reach definite evidence of such recognition we have investigated a highly curved DNA fragment to amplify this effect.A periodic recurrence of A-tracts phased with the B-DNA periodicity, like tracts of three to six adenine steps centered approximately every 10.5 base pairs, drives extended intrinsic curvatures along a DNA chain (4). These curved DNA tracts give rise to planar or quasi planar superstructures. It is worth noting that, when these short stretches of adenine steps are positioned on the same strand, the adenine bases tend to be preferentially positioned on one side of the curvature plane, while the complementary thymines will be found on the other side. When deposited on a flat surface, these curved DNA tracts will thus interact with that surface, on average, with either an A-or a T-rich face. A tract of DNA with very extensive phasing of repeated A-tracts is readily available in the kinetoplast DNA of the Trypanosomatidae protozoan Crithidia fasciculata. This DNA segment is the most highly curved DNA we know of at present (5). The profiles assumed by these curved DNA molecules on deposition on a flat surface can be observed by scanning force microscopy (SFM). Only if the sequence orientation for each of the imaged molecules could be known would it be possible to determine the face with which the molecules adsorb on the surface (on average) through the study of the average chain-curvature of the molecule ensemble. To remo...

Get a grip! The strategy of single‐molecule mechanical unfolding of multimodular proteins is extended to the investigation of any chemical bond by an appropriately tailored, comblike polymer bearing multiple instances of any desired chemical interaction or bond to be studied along its linear chain. This approach is used to study the Ni‐(His)n‐NTA complex (see picture: red, Ni‐NTA; green, His tag; His=histidine; NTA=nitrilotriacetate).

Scanning force microscopy was used to study in fluid the conformational fluctuations of two double-stranded DNA molecules resulting from differently cut pBR322 circular DNAs. A new approach was conceived to monitor the thermodynamic equilibrium of the chain dynamics on different scale lengths. This method made it possible to demonstrate that both the observed DNA molecules were allowed to equilibrate only on their local small-scale dynamics during the time of the experiment. This capability of monitoring the length scale and the time scale of the equilibration processes in the dynamics of a DNA chain is relevant to give an insight in the thermodynamics of the DNA binding with proteins and synthetic ligands. It was also shown that the small-scale equilibration of the DNA chain during surface-restricted dynamics is enough to allow a valid measurement of the local sequence-dependent curvature.

Hybrid organic-inorganic nanocomposites were prepared starting from alpha,omega-triethoxysilane-terminated poly(d,l-lactic acid) (PDLLA) to be used as potential radiopaque biocompatible coatings for medical devices. The synthesis of the organic phase precursors of given chain length was achieved via anionic polymerization of d,l-lactide using a bifunctional initiator and subsequent triethoxysilane functionalization of the end groups. PDLLA-based ceramers (ceramic polymers) were then synthesized by the sol-gel process at room temperature (rt) in the presence of different amounts of tetraethoxysilane. The rt-synthesized hybrids were then cured (at 80 or 130 degrees C), and their thermal and viscoelastic properties were investigated. All obtained hybrids were optically transparent, due to the nanometric dimension of the silica particles, and yielded clearly contrasted radiographic images.