Replicative helicase loaders: ring breakers and ring makers

Helicase loaders transfer the ring-shaped replicative helicases onto DNA. They assort into two classes: ring breakers, which place stabile hexameric rings on DNA via transient gaps at subunit interfaces; and helicase makers, which assemble hexameric rings around DNA from monomeric helicase units.     

The ring between ring fingers (RBR) protein family

Proteins of the ring between ring fingers (RBR)-domain family are characterized by three groups of specifically clustered (typically eight) cysteine and histidine residues. Whereas the amino-terminal ring domain (N-RING) binds two zinc ions and folds into a classical cross-brace ring finger, the carboxy-terminal ring domain (C-RING) involves only one zinc ion. The three-dimensional structure of the central ring domain, the IBR domain, is still unsolved. About 400 genes coding for RBR proteins have been identified in the genomes of uni- and multicellular eukaryotes and some of their viruses, but the family has not been found in archaea or bacteria. The RBR proteins are classified into 15 major subfamilies (besides some orphan cases) by the phylogenetic relationships of the RBR segments and the conservation of their sequence architecture. The RBR domain mediates protein-protein interactions and a subset of RBR proteins has been shown to function as E3 ubiquitin ligases. RBR proteins have attracted interest because of their involvement in diseases such as parkinsonism, dementia with Lewy bodies, and Alzheimer's disease, and in susceptibility to some intracellular bacterial pathogens. Here, we present an overview of the RBR-domain containing proteins and their subcellular localization, additional domains, function, specificity, and regulation.     

Constriction ring dystocia

A case of constriction ring dystocia in a 40-year-old multiparous white woman is described. She was postmature; the fetus occupied an unstable lie for which no cause could be demonstrated clinically or radiologically; during the course of an inert labour every third fetal heart sound was abnormal. At cesarean section it appeared that no lower uterine segment had formed and extreme thickness of the myometrium was encountered.     

The contractile ring

Summary Techniques of individual cell selection and precise ultramicrotomy have been employed to demonstrate that the contractile ring of cleaving HeLa cells is a transitory cytoplasmic organelle of distinctive fine structure and location. The contractile ring is an uninterrupted annulus encircling the equator of dividing cells exactly where the cleavage furrow forms. It is about 10 microns wide, up to 0.2 microns in thickness, and is composed exclusively of circumferentially aligned thin filaments 40–70 Å in diameter. Contractile ring filaments appear to be associated with the overlying plasma membrane.Controlled experiments with a mold metabolite (cytochalasin B) reveals that within a few minutes the drug abolishes the ability of HeLa cells to undergo cytokinesis. Cytochalasin B seems to decompose the contractile ring. It has no other clearly identifiable effects on other cell structures, notably the mitotic apparatus. Cytochalasin B is the only drug known which selectively inhibits cytokinesis in animal cells.In conclusion, the contractile ring is the most likely organelle responsible for cytokinesis in HeLa cells. Similar organelles probably occur in all cleaving animal cells. Successful cleavage depends upon the structural and functional integrity of the contractile ring.     

Dearomatizing benzene ring reductases

The high resonance energy of the benzene ring is responsible for the relative resistance of aromatic compounds to biodegradation. Nevertheless, bacteria from nearly all physiological groups have been isolated which utilize aromatic growth substrates as the sole source of cell carbon and energy. The enzymatic dearomatization of the benzene nucleus by microorganisms is accomplished in two different manners. In aerobic bacteria the aromatic ring is dearomatized by oxidation, catalyzed by oxygenases. In contrast, anaerobic bacteria attack the aromatic ring by reductive steps. Key intermediates in the anaerobic aromatic metabolism are benzoyl-CoA and compounds with at least two meta-positioned hydroxyl groups (resorcinol, phloroglucinol and hydroxyhydroquinone). In facultative anaerobes, the reductive dearomatization of the key intermediate benzoyl-CoA requires a stoichiometric coupling to ATP hydrolysis, whereas reduction of the other intermediates is readily achieved with suitable electron donors. Obligately anaerobic bacteria appear to use a totally different enzymology for the reductive dearomatization of benzoyl-CoA including selenocysteine- and molybdenum- containing enzymes.     

Single-walled tubulin ring polymers

An unusual class of nanoscopic, ring-shaped, single-walled biopolymers arises when alphabeta-tubulin is mixed with certain small peptides obtained from various marine organisms and cyanobacteria. The single-ring structures, whose mean molecular weight depends on the specific peptide added to the reaction mixture, usually have sharp mass distributions corresponding, e.g., to rings containing eight tubulin dimers (when the added peptide is cryptophycin) and 14 dimers (e.g., with dolastatin). Although the ring-forming peptides have been shown to possess antimitotic properties when tested with cultured eukaryotic cells (and thus have generated considerable interest as possible agents to be used in the treatment of cancer), it is not our intention to extensively discuss the potential pharmacological properties of the peptides. Rather, we will review the polymeric structures that form and illustrate how certain physical techniques can be used to characterize their properties and interactions. The nanoscopic size and particular geometry of the individual rings make them appropriate targets for scattering and hydrodynamic techniques that provide details about their structure in solution, but it is necessary to relate measured data to postulated structures by nontrivial, albeit straight-forward, mathematical, and computational means. We will discuss how this is done when one uses such methods as small angle neutron scattering, dynamic light scattering, fluorescence correlation spectroscopy, and sedimentation velocity measurements. Moreover, we show that, by using several techniques, one can eliminate degeneracy to provide better discrimination between model structures.     

Counterion-induced actin ring formation

Actin filaments form rings and loops when > 20 mM divalent cations are added to very dilute solutions of phalloidin-stabilized filamentous actin (F-actin). Some rings consist of very long single actin filaments partially overlapping at their ends, and others are formed by small numbers of filaments associated laterally. In some cases, undulations of the rings are observed with amplitudes and dynamics similar to those of the thermal motions of single actin filaments. Lariat-shaped aggregates also co-exist with rings and rodlike bundles. These polyvalent cation-induced actin rings are analogous to the toroids of DNA formed by addition of polyvalent cations, but the much larger diameter of actin rings reflects the greater bending stiffness of F-actin. Actin rings can also be formed by addition of streptavidin to crosslink sparsely biotinylated F-actin at very low concentrations. The energy of bending in a ring, calculated from the persistence length of F-actin and the ring diameter, provides an estimate for the adhesion energy mediated by the multivalent counterions, or due to the streptavidin-biotin bonds, required to keep the ring closed.     

Closing the ring: historical biogeography of the salamander ring species Ensatina eschscholtzii

Aim The salamander Ensatina eschscholtzii Gray is a classic example of a ring species, or a species that has expanded around a central barrier to form a secondary contact characterized by species‐level divergence. In the original formulation of the ring species scenario, an explicit biogeographical model was proposed to account for the occurrence of intraspecific sympatry between two subspecies in southern California (the ‘southern closure’ model). Here we develop an alternative ring species model that is informed by the geomorphological development of the California Coast Ranges, and which situates the point of ring closure in the Monterey Bay region of central coastal California (the ‘Monterey closure’ model). Our study has two aims. The first is to use phylogenetic methods to evaluate the two competing biogeographical models. The second is to describe patterns of phylogeographical diversity throughout the range of the Ensatina complex, and to compare these patterns with previously published molecular systematic data. Location Western North America, with a focus on the state of California, USA. Methods We obtained mitochondrial DNA sequence data from 385 individuals from 224 populations. A phylogeny was inferred using Bayesian techniques, and the geographical distributions of haplotypes and clades were mapped. The two biogeographical ring species models were tested against our Bayesian topology, including the associated Bayesian 95% credible set of trees. Results High levels of phylogeographical diversity were revealed, especially in central coastal and northern California. Our Bayesian topology contradicts the Monterey closure model; however, 0.08% of the trees in our Bayesian 95% credible set are consistent with this model. In contrast, the classic ring species biogeographical model (the southern closure model) is consistent with our Bayesian topology, as were 99.92% of the trees in our 95% credible set. Main conclusions Our Bayesian phylogenetic analysis most strongly supports the classic ring species model, modified to accommodate an improved understanding of the complex geomorphological evolution of the California Coast Ranges. In addition, high levels of phylogeographical diversity in central and northern California were identified, which is consistent with the striking levels of allozymic differentiation reported previously from those regions.     

Mechanisms of ring chromosome formation in 11 cases of human ring chromosome 21

We studied the mechanism of ring chromosome 21 (r(21)) formation in 13 patients (11 unique r(21)s), consisting of 7 from five families with familial r(21) and 6 with de novo r(21). The copy number of chromosome 21 sequences in the rings of these patients was determined by quantitative dosage analyses for 13 loci on 21q. Nine of 11 r(21)s, including the 5 familial r(21)s, showed no evidence for duplication of 21q sequences but did show molecular evidence of partial deletion of 21q. These data were consistent with the breakage and reunion of short- and long-arm regions to form the r(21), resulting in deletion of varying amounts of 21q22.1 to 21qter. The data from one individual who had a Down syndrome phenotype were consistent with asymmetric breakage and reunion of 21q sequences from an intermediate isochromosome or Robertsonian translocation chromosome as reported by Wong et al. Another patient, who also exhibited Down syndrome, showed evidence of a third mechanism of ring formation. The likely initial event was breakage and reunion of the short and long arms, resulting in a small r(21), followed by a sister-chromatid exchange resulting in a double-sized and symmetrically dicentric r(21). The phenotype of patients correlated well with the extent of deletion or duplication of chromosome 21 sequences. These data demonstrate three mechanisms of r(21) formation and show that the phenotype of r(21) patients varies with the extent of chromosome 21 monosomy or trisomy.     

Presence of telomeric and subtelomeric sequences at the fusion points of ring chromosomes indicates that the ring syndrome is caused by ring instability

In situ hybridization of a telomeric (TTA-GGG) _n sequence to metaphases from three cases of ring chromosome, involving respectively chromosomes 4, 16, and 20, showed the presence of the cognate sequences in all three rings. To investigate whether these ring chromosomes originated by telomere-telomere fusion, we determined, by in situ hybridization, whether telomere-associated sequences and/or specific distal sequences were still present in the ring chromosomes. The finding that these sequences were preserved in all the ring chromosomes strongly indicates that they originated by telomere-telomere fusion. All three subjects carrying the ring chromosomes are affected by the so-called ring syndrome, with failure to thrive, minor dysmorphic signs and no major anomalies. The r(4) patient has the ring in mosaic form with a normal cell line and has normal intelligence. The r(16) and the r(20) patients have moderate mental retardation and suffer from seizures. We conclude that the ring syndrome, even in its more severe manifestation, is caused by ring chromosome instability.