Transcarboxylase 12S crystal structure: hexamer assembly and substrate binding to a multienzyme core

Transcarboxylase from Propionibacterium shermanii is a 1.2 MDa multienzyme complex that couples two carboxylation reactions, transferring CO(2)(-) from methylmalonyl-CoA to pyruvate, yielding propionyl-CoA and oxaloacetate. The 1.9 A resolution crystal structure of the central 12S hexameric core, which catalyzes the first carboxylation reaction, has been solved bound to its substrate methylmalonyl-CoA. Overall, the structure reveals two stacked trimers related by 2-fold symmetry, and a domain duplication in the monomer. In the active site, the labile carboxylate group of methylmalonyl-CoA is stabilized by interaction with the N-termini of two alpha-helices. The 12S domains are structurally similar to the crotonase/isomerase superfamily, although only domain 1 of each 12S monomer binds ligand. The 12S reaction is similar to that of human propionyl-CoA carboxylase, whose beta-subunit has 50% sequence identity with 12S. A homology model of the propionyl-CoA carboxylase beta-subunit, based on this 12S crystal structure, provides new insight into the propionyl-CoA carboxylase mechanism, its oligomeric structure and the molecular basis of mutations responsible for enzyme deficiency in propionic acidemia.     

CONSPECIFIC SPERM PRECEDENCE IS AN EFFECTIVE BARRIER TO HYBRIDIZATION BETWEEN CLOSELY RELATED SPECIES

Conspecific sperm precedence is widespread in animals, appears to evolve rapidly, and is thought to have the potential to prevent hybridization between closely related species. However, to date no study has tested the isolating potential of such a barrier in mixed populations of two taxa under conditions in which other potential barriers to gene flow are controlled for or are prevented from operating. We tested the isolating potential of conspecific sperm precedence in the ground crickets Allonemobius fasciatus and A. socius in population cage experiments in which the frequency of the two species was varied. Despite the observation of abundant interspecific matings, the proportions of hybrid progeny were low and differed statistically from the proportions expected in the absence of conspecific sperm precedence. The results demonstrate that conspecific sperm precedence can severely limit gene flow between closely related species, even when one species is less abundant than the other.     

Ligand connected metal clusters. The molecular structures and solid state packing of {Ru3(CO)11}2(bis(diphenylphosphino)ethane) and {Ru3(CO)11}2(1,4-bis(diphenylphosphino)benzene)

The preparation and structural characterization of {Ru₃(CO)₁₁}₂(1,4-bis(diphenylphosphino)benzene), a modified synthesis of 1,4-bis(diphenylphosphino)benzene, and the structural characterization of {Ru₃(CO)₁₁}₂(bis(diphenylphosphino)ethane) are reported. In both compounds two metal cluster units are connected through ditertiary-phosphine ligands. Both molecules consist of centrosymmetric units in which the diphosphine ligands are largely covered by the triangular ruthenium clusters. No direct interaction between the two cluster units occurs within individual molecules. Molecular packing in the solid state is dominated by interactions between sets of carbon monoxide ligands in motifs that were previously identified in the solid state structure of the parent cluster, Ru₃(CO)₁₂.     

High genetic susceptibility to ethanol withdrawal predicts low ethanol consumption

C57BL/6J (B6) inbred mice are well known to drink large amounts of alcohol (ethanol) voluntarily and to have only modest ethanol-induced withdrawal under fixed dose conditions. In contrast, DBA/2J (D2) mice are ``teetotallers' and exhibit severe ethanol withdrawal. Speculation that an inverse genetic relationship existed between these two traits was substantiated by meta-analysis of existing data collected in multiple genetic models, including large panels of standard and recombinant inbred strains, their crosses, and selectively bred mouse lines. Despite methodological differences among laboratories in measurement of both preference drinking and withdrawal, a nearly universal finding was that genotypes consuming large amounts of 10% ethanol (calculated as g/kg/day) during two-bottle choice preference drinking were genetically predisposed to low withdrawal scores in independent studies after either acute or chronic ethanol treatment. Conversely, low-drinking genotypes had higher withdrawal severity scores. The genetic relationship appears to be strongest in populations derived from B6 and D2, where data from more genotypes (BXD RIs, B6D2F₂s, BXD RI F₁s, and B6D2F₂-derived selectively bred lines) were available for analysis. Gene mapping studies in these populations identified four chromosome regions [on Chromosomes (Chrs) 1, 2, 4, and 15] where genes might potentially influence both traits. Among genotypes with greater genetic diversity (for example, a panel of standard inbred strains or selectively bred lines), the relationship was less pronounced. Thus, reduced susceptibility to the development of high alcohol use may be supported by increased genetic susceptibility to ethanol withdrawal symptoms. Received: 15 September 1998 / Accepted: 8 October 1998     

TGFβ3 and loading increases osteocyte survival in human cancellous bone cultured ex vivo

The goal of this study was to assess the effect of the addition of TGFβ₃, alone or in combination with loading, on the survival of osteocytes in 3D human explant cancellous bone during long-term culture in an ex vivo loading bioreactor. Human cancellous bone explants were cultured for up to 14 days with or without TGFβ₃ (15 ng ml⁻¹) and with or without loading (300 cycles, at 1 Hz, producing 4000 microstrain). Bone core response was visualized using undecalcified histology with morphological methods after embedding with Technovit 9100 New® resin. Histological examination revealed normal gross level bone structure with or without the application of load or the addition of TGFβ₃. The viability of the osteocytes within the bone was assessed by lactate dehydrogenase (LDH) activity. We demonstrate that this ex vivo loading bioreactor is able to maintain a high percentage (over 50%) of viable osteocytes throughout the bone explants after 14 days in ex vivo culture. Further to this, the combination of daily loading and TGFβ₃ administration produced superior osteocyte survival at the core centres when compared to loading or TGFβ alone. Copyright © 2008 John Wiley & Sons, Ltd.     

Identifying high-level salt marshes using a palynomorphic fingerprint with potential implications for tracking sea level change

Palynomorph assemblages from South Carolina salt marshes were analyzed from high marsh, low marsh, and salt panne zones. High marsh zones, dominated by Juncus roemerianus, consistently had a distinctive palynomorphic signature, whereas this was not the case for assemblages from low marsh and salt panne zones.The Juncus environment was distinguishable from all others by the following parameters: 1) higher palynomorph diversity (nearly double the taxonomic diversity present in the other types), 2) the presence of a high percentage (over 10% of the palynomorph count) of Fungal Spore Type A, and 3) the presence of Atrotorquata lineata, a fungal species that was found solely in sediments beneath Juncus marshes. Locating the positions of Juncus environments in cores should allow one to trace the rise and fall of sea level over time.