Ru, Pd, Ir, Pt and Ag in two Cretaceous-Tertiary boundary Danish samples and in the Orgueil carbonaceous chondrite were determined by isotope dilution employing the inductively coupled-plasma mass spectrometer. Some mutual abundance fractionation potentially reflecting the difference in metal density can be recognized among the platinum-group elements (PGE). Silver is anomalously enriched by a factor of 15 compared with PGE, on the basis of “cosmic” abundances. This enrichment of Ag can be interpreted as indicating the precipitation of silver sulfide from seawater. Sulfate reduction to sulfide anion in seawater is suggested as an explanation of anomalous enrichment of Ag and Re.
Four parabolic functions were derived separately for the first, the second, the third and the fourth tetrads of lanthanides in samples of leuco-granitic gneiss from Korea, and seawater from off the coasts of Shimane and Kochi prefectures, Japan. We conclude that relative lanthanide abundances in each of those samples are mathematically expressed by a set of four parabolic functions. The parabolic functions are applicable to the abundance ratio values as well as the values of aberration from the cusp-spanning lines. Both parabolic function systems are significant in different senses, but a set of four parabolas for “aberration” is convenient in visualizing the characteristics of lanthanide tetrad phenomenon observed for the sample. Difference in those characteristics can be interpreted to reflect uniquely the state of lanthanides present and/or the condition of genesis of the sample.
A possible Zn-binding structure of a plant transcription factor, AOBP, and its mode of DNA-binding are discussed in reference to crystal/solution structures of steroid hormone receptors and GATA1. The amino acid sequence of AOBP (aa36-aa78) has five Cys residues. Cys[aa42] and Cys[aa45] are positioned with the same spacing as those found in the loop region of the reference proteins, while Cys[aa67] and Cys[aa70] in the same spacing as the other two Cys residues in the recognition helix. These, and some other features of the AOBP sequence suggest that this part of the protein folds into a Zn-binding domain, which corresponds to half the DNA-binding domain of a steroid hormone receptor and thereby binds to the target DNA site through an α-helix. Similar sequences are found in several other plant proteins and the evolutionary relation of these proteins and steroid hormone receptors is discussed.
Chimeric proteins from chicken and yeast calmodulin were prepared, and roles of three structural domains, N-domain (1-72), central helix (73-87) and C-domain (88-148), were evaluated. Mutants with the chicken-type C-domain activated the cyclic nucleotide phosphodiesterase with small values of Kact (the concentration of calmodulin giving a half maximal activation), a property of chicken calmodulin. On the other hand mutants with the yeast-type C-domain activated the phosphodiesterase with μM range of Kact, a property of yeast calmodulin. In activation of myosin light chain kinase, introduction of the yeast-type C-domain into chicken calmodulin increased the Kact value by more than 1000-fold with a dramatic decrease in the maximum activity (Vmax). On the other hand introduction of the chicken-type C-domain led to a profile with lower Kact and higher Vmax. Minor effects on Vmax and Kact were observed by substitution of the central helix. Although various small contributions of the N-domain were observed, a common role of the chicken-type C-domain was suggested to catch and maintain the high-affinity interaction with target enzymes.