In response to adverse environmental factors, Escherichia coli cells actively produce Dps proteins which form ordered complexes (biocrystals) with bacterial DNA to protect the genome. The effect of biocrystallization has been described extensively in the scientific literature; furthermore, to date, the structure of the Dps–DNA complex has been established in detail in vitro using plasmid DNA. In the present work, for the first time, Dps complexes with E. coli genomic DNA were studied in vitro using cryo-electron tomography. We demonstrate that genomic DNA forms one-dimensional crystals or filament-like assemblies which transform into weakly ordered complexes with triclinic unit cells, similar to what is observed for plasmid DNA. Changing such environmental factors as pH and KCl and MgCl2 concentrations leads to the formation of cylindrical structures.

Rapid increase of intracellular synthesis of specific histone-like Dps protein that binds DNA to protect the genome against deleterious factors leads to in cellulo crystallization-one of the most curious processes in the area of life science at the moment. However, the actual structure of the Dps-DNA co-crystals remained uncertain in the details for more than two decades. Cryo-electron tomography and small-angle X-ray scattering revealed polymorphous modifications of the co-crystals depending on the buffer parameters. Two different types of the Dps-DNA co-crystals are formed in vitro: triclinic and cubic. Three-dimensional reconstruction revealed DNA and Dps molecules in cubic co-crystals, and the unit cell parameters of cubic lattice were determined consistently by both methods.

Under severe or prolonged stress, bacteria produce a nonspecific DNA-binding protein (Dps), which effectively protects DNA against damaging agents both in vitro and in vivo by forming intracellular biocrystals. The phenomenon of protective crystallization of DNA in living cells has been intensively investigated during the last two decades; however, the results of studies are somewhat contradictory, and up to now, there has been no direct determination of a Dps-DNA crystal structure. Here, we report the in vitro analysis of the vital process of Dps-DNA co-crystallization using two complementary structural methods: synchrotron small-angle X-ray scattering in solution and cryo-electron tomography. Importantly, for the first time, the DNA in the co-crystals was visualized, and the lattice parameters of the crystalline Dps-DNA complex were determined.

Amphipathic secondary structure elements and putative cholesterol recognizing amino acid consensus (CRAC) motifs as governing factors of highly specific matrix protein interactions with raft-type membranes in enveloped viruses Victor A. Radyukhin*, Liubov A. Dadinova, Ivan A. Orlov and Ludmila A. Baratova Belozersky Institute of Physico-Chemical Biology, Moscow State University, Moscow, Russia; A.V. Shubnikov Institute of Crystallography of Federal Scientific Research Centre ‘Crystallography and Photonics’ of Russian Academy of Sciences, Moscow, Russia; Joint Institute for Nuclear Research, Dubna, Russia