A statistical survey of the torsion angles, bond angles, and bond lengths in the sugar and phosphate groups of well-refined mononucleoside, mononucleotide, dinucleoside monophosphate, and trinucleoside diphosphate crystal structures contained in the Cambridge Structural Database and the Nucleic Acid Database is reported. The mean values of the geometrical parameters in these structures and their estimated standard deviations are separated according to their chemistry and conformation. These new parameters serve as a basis for a dictionary of standard nucleic acid geometry.

Structures at atomic resolution (up to 1.0,&) which contain bases, sugars or the phosphodiester linkage, were selected from the Nucleic Acid Database or the Cambridge Structural Database to build a nucleic acid dictionary from X-ray refined structures. The dictionary consists of the average values for bond distances, bond angles and dihedral angles. The variance of the sample is used to provide information about the expected r.m.s, deviations of the refined parameters. A dictionary was constructed for refinement trials in X-PLOR. The dictionary includes RNA and DNA in C2'-endo and C3'-endo sugar pucker conformations, as well as values for the backbone dihedrals. Tests were performed on the dictionary using three structures: a B-DNA, a Z-DNA and a protein-DNA complex. During the course of refinement, all three structures showed significant improvements as measured by r.m.s, deviations and R factors when compared to the previous DNA dictionary.

We present estimates of the bond-length and bond-angle parameters for the nitrogenous base side groups of nucleic acids. These values are the result of a statistical survey of small molecules in the Cambridge Structural Database for which high-resolution X-ray and neutron crystal structures are available. The statistics include arithmetic means and standard deviations for the different samples, as well as comparisons of the population distributions for sugar-and non-sugar-derivatized bases. These accumulated data provide appropriate target values for refinements of oligonucleotide structures, as well as sets of standard atomic coordinates for the five common bases.

The classification feast͞famine regulatory proteins (FFRPs) encompasses archaeal DNA-binding proteins with Escherichia coli transcription factors, the leucine-responsive regulatory protein and the asparagine synthase C gene product. In this paper, we describe two forms of the archaeal FFRP FL11 (pot0434017), both assembled from dimers. When crystallized, a helical cylinder is formed with six dimers per turn. In contrast, in solution, disks are formed, most likely consisting of four dimers each; an observation by cryoelectron microscopy. Whereas each dimer binds a 13-bp sequence, different forms will discriminate between promoters, based on the numbers of repeating 13-bp sequences, and types of linkers inserted between them, which are either of 7-8 or Ϸ18 bp. The amino acid sequences of these FFRPs are designed to form the same type of 3D structures, and the transition between their assembly forms is regulated by interaction with small molecules. These considerations lead us to propose a possible mechanism for regulating a number of genes by varying assembly forms and by combining different FFRPs into these assemblies, responding to environmental changes.