Single molecular observation of DNA and DNA complexes by atomic force microscopy.

Atomic force microscopy (AFM) provides a novel way to understand the structure-function relationship of protein synthesis at a single-molecular level. High-resolution AFM imaging in air, liquid and vacuum allows for single DNA, RNA and proteins to be observed at the nano-scale in addition to their conformational transitions, bound states, temporal behavior, and assembly. The recent development of frequency modulation mode AFM has led to imaging hydration structures of DNA in water and molecular polarization of DNA complexes in vacuum. Real-time imaging in near-physiological environments captures transcriptional activation with characteristic conformation of DNA-protein complexes. We review current achievements and the future potential of methodological and biological AFM to reveal insights into DNA, RNA and their complexes.

Profiling of gene-dependent translational progress in cell-free protein synthesis by real-space imaging.

In general, gene-dependent translational progress affects the efficiency of protein expression. To evaluate the translational progress of protein synthesis, it is necessary to trace the time course of translation as well as the quantity of products. Here we present a new method for tracking translation steps in cell-free protein synthesis using atomic force microscopy (AFM). The cell-free protein synthesis system is useful to track the inherent translational progress of a target gene, whereas conventional UV absorption measurement coupled with density gradient fractionation is difficult to analyze such small sample quantities. Because the high resolution of AFM enables us to clearly count the number of ribosomes included in polysomes, polysome profiles can be obtained directly without complicated fractionation. With this method, we could elucidate the detailed polysome profile with only 1 microl of sample solution. We observed the translational progress of green fluorescent protein synthesis, a model of high-expression protein, as well as human retinoid X receptor. Detailed polysome profiles showed different patterns of translational progress and were clearly associated with the results of time-dependent protein expression. Our study suggests the possibility for comprehensive character analysis of inherent gene-dependent translational progress.

Electrostatic force microscopy: imaging DNA and protein polarizations one by one.

We present electrostatic force microscopy images of double-stranded DNA and transcription complex on an insulating mica substrate obtained with molecular resolution using a frequency-mode noncontact atomic force microscope. The electrostatic potential images show that both DNA and transcription complexes are polarized with an upward dipole moment. Potential differences of these molecules from the mica substrate enabled us to estimate dipole moments of isolated DNA and transcription complex in zero external field to be 0.027 D/base and 0.16 D/molecule, respectively. Scanning capacitance microscopy demonstrates characteristic contrast inversion between DNA and transcription complex images, indicating the difference in electric polarizability of these molecules. These findings indicate that the electrostatic properties of individual biological molecules can be imaged on an insulator substrate while retaining complex formation.

Length-dependent recognition of double-stranded ribonucleic acids by retinoic acid-inducible gene-I and melanoma differentiation-associated gene 5.

The ribonucleic acid (RNA) helicases retinoic acid-inducible gene-I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5) recognize distinct viral and synthetic RNAs, leading to the production of interferons. Although 5'-triphosphate single-stranded RNA is a RIG-I ligand, the role of RIG-I and MDA5 in double-stranded (ds) RNA recognition remains to be characterized. In this study, we show that the length of dsRNA is important for differential recognition by RIG-I and MDA5. The MDA5 ligand, polyinosinic-polycytidylic acid, was converted to a RIG-I ligand after shortening of the dsRNA length. In addition, viral dsRNAs differentially activated RIG-I and MDA5, depending on their length. Vesicular stomatitis virus infection generated dsRNA, which is responsible for RIG-I-mediated recognition. Collectively, RIG-I detects dsRNAs without a 5'-triphosphate end, and RIG-I and MDA5 selectively recognize short and long dsRNAs, respectively.