New evidence for the Ontong Java Nui hypothesis.

The formation of the Ontong Java Nui super oceanic plateau (OJN), which is based on the model that the submarine Ontong Java Plateau (OJP), Manihiki Plateau (MP), and Hikurangi Plateau (HP) were once its contiguous fragments, could have been the largest globally consequential volcanic event in Earth's history. This OJN hypothesis has been debated given the paucity of evidence, for example, the differences in crustal thickness, the compositional gap between MP and OJP basalts and the apparent older age of both plateaus relative to HP remain unresolved. Here we investigate the geochemical and 40Ar-39Ar ages of dredged rocks recovered from the OJP's eastern margin. Volcanic rocks having compositions that match the low-Ti MP basalts are reported for the first time on the OJP and new ~ 96-116 Ma and 67-68 Ma 40Ar-39Ar age data bridge the temporal gap between OJP and HP. These results provide new evidence for the Ontong Java Nui hypothesis and a framework for an integrated tectonomagmatic evolution of the OJP, MP, and HP. The isotopic data imply four mantle components in the source of OJN that are also expressed in present-day Pacific hotspots sources, indicating origin from (and longevity of) the Pacific Large Low Shear-wave Velocity Province.

An extraterrestrial trigger for the Early Cretaceous massive volcanism? Evidence from the paleo-Tethys Ocean.

The Early Cretaceous Greater Ontong Java Event in the Pacific Ocean may have covered ca. 1% of the Earth's surface with volcanism. It has puzzled scientists trying to explain its origin by several mechanisms possible on Earth, leading others to propose an extraterrestrial trigger to explain this event. A large oceanic extraterrestrial impact causing such voluminous volcanism may have traces of its distal ejecta in sedimentary rocks around the basin, including the paleo-Tethys Ocean which was then contiguous with the Pacific Ocean. The contemporaneous marine sequence at central Italy, containing the sedimentary expression of a global oceanic anoxic event (OAE1a), may have recorded such ocurrence as indicated by two stratigraphic intervals with (187)Os/(188)Os indicative of meteoritic influence. Here we show, for the first time, that platinum group element abundances and inter-element ratios in this paleo-Tethyan marine sequence provide no evidence for an extraterrestrial trigger for the Early Cretaceous massive volcanism.

Determinants of the DNA-binding specificity of the Avian homeodomain protein, AKR.

AKR (Avian Knotted-Related) was the first example of a vertebrate homeodomain protein with a highly divergent Ile residue at position 50 of the DNA-recognition helix. The protein was cloned from a liver cDNA expression library of a day-9 chick embryo by virtue of its ability to bind to the F' site in the proximal promoter of the avian apoVLDLII gene. Expression of the apoVLDLII gene is completely estrogen dependent, and mutation or deletion of the F' site decreases estrogen inducibility 5- to 10-fold. Subsequent data indicated that AKR is capable of repressing the hormone responsiveness of the apoVLDLII promoter, specifically through binding to F'. Involvement of the F' site in the hormone-dependent activation of apoVLDLII gene expression, as well as AKR-mediated repression, strongly suggests that both positive and negative regulatory factors interact with this site. Although several mammalian proteins have now been isolated whose homeodomains share many of the structural features of AKR, including the Ile at position 50, little is known of their functions in vivo or the identities of the genes they regulate. Consequently, the elements through which they exert their effects and the structural determinants of their binding specificities remain largely uncharacterized. In this study, we defined the sequence specificity of binding by AKR using polymerase chain reaction-assisted optimal site selection and determined the affinity with which the protein binds to both the optimized site and the F' site. Additionally, we generated a three-dimensional model of the AKR homeodomain binding to its optimized site and probed the validity of the model by examining the consequences of mutating amino acid residues in recognition helix 3 and the N-terminal arm on the binding specificity of the homeodomain. Finally, we present evidence that the F' site itself may act as an estrogen response element (ERE) when in the vicinity of imperfect or canonical EREs and that AKR can repress hormone inducibility mediated via this site.

Isolation and characterization of the chicken homeodomain protein AKR.

Expression of the avian apoVLDLII gene is liver specific and completely dependent on estrogen. Previous analyses of protein binding sites in the apoVLDLII promoter revealed interactions between liver-enriched and ubiquitous factors at a location, site F', between nucleotides -229 and -260 relative to the major transcriptional start site. Site-directed mutagenesis of G residues contacted by these factors decreased expression from the promoter approximately 5-fold in the avian hepatoma cell line LMH2A. We have used this site to screen a cDNA expression library constructed from day 9 embryonic liver RNA. One of the two DNA binding factors isolated is a novel homeodomain protein. With the exception of the homeodomain itself, which is atypically located close to the protein N-terminus, the factor displays little similarity to any known DNA binding protein. Its homeodomain is most similar to that of the maize protein Knotted-1, while the most closely related vertebrate domain is that of the human proto-oncoprotein Pbx1. We demonstrate that the DNA binding specificity of the factor is consistent with its involvement in the ubiquitous complex formed with site F' and that it is capable of suppressing expression from the apoVLDLII promoter in short-term transfection experiments. These studies, combined with its DNA binding specificity, the tissue distribution of its mRNA and its developmental regulation, suggest a role as a negative regulator of gene expression in non-hepatic tissues and in the liver early during embryogenesis.

Cloning of an avian antithrombin: developmental and hormonal regulation of expression.

We present the complete amino acid sequence of a non-mammalian antithrombin together with a structural comparison to its mammalian homologs. In the majority of cases, but not all, the analyses indicate high degrees of primary structural identity within regions of predicted or demonstrated functional importance. We have also taken advantage of the avian model to define the ontogeny of antithrombin mRNA expression. Northern analyses indicate chicken antithrombin mRNA levels to be detectable in the liver early during development and increase to reach adult levels just prior to hatching. Although no sex-dependent differences in antithrombin mRNA levels were detected, the administration of estrogen to male roosters was observed to cause a 3-fold decrease in antithrombin mRNA levels, suggesting that the hormone may alter circulating antithrombin levels via a pre-translational mechanism.