Gravity and the orientation of cell division

A novel culture system for mammalian cells was used to investigate division orientations in populations of Chinese hamster ovary cells and the influence of gravity on the positioning of division axes. The cells were tethered to adhesive sites, smaller in diameter than a newborn cell, distributed over a nonadhesive substrate positioned vertically. The cells grew and divided while attached to the sites, and the angles and directions of elongation during anaphase, projected in the vertical plane, were found to be random with respect to gravity. However, consecutive divisions of individual cells were generally along the same axis or at 90° to the previous division, with equal probability. Thus, successive divisions were restricted to orthogonal planes, but the choice of plane appeared to be random, unlike the ordered sequence of cleavage orientations seen during early embryo development.

Orientation of the transcription preinitiation complex in Archaea

The basal transcription machinery of Archaea corresponds to the minimal subset of factors required for RNA polymerase II transcription in eukaryotes. Using just two factors, Archaea recruit the RNA polymerase to promoters and define the direction of transcription. Notably, the principal determinant for the orientation of transcription is not the recognition of the TATA box by the TATA-box-binding protein. Instead, transcriptional polarity is governed by the interaction of the archaeal TFIIB homologue with a conserved motif immediately upstream of the TATA box. This interaction yields an archaeal preinitiation complex with the same orientation as the analogous eukaryal complex.

The orientation of the AP-1 heterodimer on DNA strongly affects transcriptional potency

Activation of gene transcription in eukaryotes requires the cooperative assembly of an initiation complex containing many protein subunits. The necessity that these components contact each other and the promoter/enhancer in defined ways suggests that their spatial arrangement might influence the activation response. Indeed, growing evidence indicates that DNA architecture can profoundly affect transcriptional potency. Much less is known about the influence of protein architecture on transcriptional activation. Here, we examine the architectural dependence of activator function through the analysis of matched pairs of AP-1•DNA complexes differing only in their orientation. Mutation of a critical Arg residue in the basic-leucine zipper domain of either Fos or Jun yielded single point-mutant heterodimers that bind DNA in a single defined orientation, as determined directly by native chemical ligation/affinity cleavage; by contrast, the corresponding wild-type protein binds DNA as a roughly equal mixture of two isomeric orientations, which are related by subunit interchange. The stereochemistry of the point-mutant heterodimers could be switched by inversion of a C•G base pair in the center of the AP-1 site, thus providing access to both fixed orientational isomers. Yeast reporter gene assays consistently revealed that one orientational isomer activates transcription at least 10-fold more strongly than the other. These results suggest that protein architecture, especially the spatial relationship of the activation domain to the promoter, can exert a powerful influence on activator potency.

Dynamics and orientation of N+(CD3)3-bromoacetylcholine bound to its binding site on the nicotinic acetylcholine receptor

Dynamic and structural information has been obtained for an analogue of acetylcholine while bound to the agonist binding site on the nicotinic acetylcholine receptor (nAcChoR), using wide-line deuterium solid-state NMR. Analysis of the deuterium lineshape obtained at various temperatures from unoriented nAcChoR membranes labeled with deuterated bromoacetylcholine (BAC) showed that the quaternary ammonium group of the ligand is well constrained within the agonist binding site when compared with the dynamics observed in the crystalline solids. This motional restriction would suggest that a high degree of complementarity exists between the quaternary ammonium group of the ligand and the protein within the agonist binding site. nAcChoR membranes were uniaxially oriented by isopotential centrifugation as determined by phosphorous NMR of the membrane phospholipids. Analysis of the deuterium NMR lineshape of these oriented membranes enriched with the nAcChoR labeled with N+(CD3)3-BAC has enabled us to determine that the angle formed between the quaternary ammonium group of the BAC and the membrane normal is 42° in the desensitized form of the receptor. This measurement allows us to orient in part the bound ligand within the proposed receptor binding site.

Centromere mapping and orientation of the molecular linkage map of rice (Oryza sativa L.).

Rice has become a model cereal plant for molecular genetic research. Rice has the most comprehensive molecular linkage maps with more than 2000 DNA markers and shows synteny and colinearity with the maps of other cereal crops. Until now, however, no information was available about the positions of centromeres and arm locations of markers on the molecular linkage map. Secondary and telotrisomics were used to assign restriction fragment length polymorphism markers to specific chromosome arms and thereby to map the positions of centromeres. More than 170 restriction fragment length polymorphism markers were assigned to specific chromosome arms through gene dosage analysis using the secondary and telotrisomics and the centromere positions were mapped on all 12 linkage groups. The orientations of seven linkage groups were reversed to fit the "short arm on top" convention and the corrected map is presented.