Imaging of nucleic acids with atomic force microscopy

Atomic force microscopy (AFM) is a key tool of nanotechnology with great importance in applications to DNA nanotechnology and to the recently emerging field of RNA nanotechnology. Advances in the methodology of AFM now enable reliable and reproducible imaging of DNA of various structures, topologies, and DNA and RNA nanostructures. These advances are reviewed here with emphasis on methods utilizing modification of mica to prepare the surfaces enabling reliable and reproducible imaging of DNA and RNA nanostructures. Since the AFM technology for DNA is more mature, AFM imaging of DNA is introduced in this review to provide experience and background for the improvement of AFM imaging of RNA. Examples of imaging different structures of RNA and DNA are discussed and illustrated. Special attention is given to the potential use of AFM to image the dynamics of nucleic acids at the nanometer scale. As such, we review recent advances with the use of time-lapse AFM.

The interaction of DiaA and DnaA regulates the replication cycle in E. coli by directly promoting ATP DnaA-specific initiation complexes

Escherichia coli DiaA is a DnaA-binding protein that is required for the timely initiation of chromosomal replication during the cell cycle. In this study, we determined the crystal structure of DiaA at 1.8 A resolution. DiaA forms a homotetramer consisting of a symmetrical pair of homodimers. Mutational analysis revealed that the DnaA-binding activity and formation of homotetramers are required for the stimulation of initiation by DiaA. DiaA tetramers can bind multiple DnaA molecules simultaneously. DiaA stimulated the assembly of multiple DnaA molecules on oriC, conformational changes in ATP-DnaA-specific initiation complexes, and unwinding of oriC duplex DNA. The mutant DiaA proteins are defective in these stimulations. DiaA associated also with ADP-DnaA, and stimulated the assembly of inactive ADP-DnaA-oriC complexes. Specific residues in the putative phosphosugar-binding motif of DiaA were required for the stimulation of initiation and formation of ATP-DnaA-specific-oriC complexes. Our data indicate that DiaA regulates initiation by a novel mechanism, in which DiaA tetramers most likely bind to multiple DnaA molecules and stimulate the assembly of specific ATP-DnaA-oriC complexes. These results suggest an essential role for DiaA in the promotion of replication initiation in a cell cycle coordinated manner.