Primary structure and evolutionary relationship between the adult alpha-globin genes and their 5'-flanking regions of Xenopus laevis and Xenopus tropicalis

To investigate the evolution of globin genes in the genus Xenopus, we have determined the primary structure of the related adult alpha I- and alpha II-globin genes of X. laevis and of the adult alpha-globin gene of X. tropicalis, including their 5'-flanking regions. All three genes are comprised of three exons and two introns at homologous positions. The exons are highly conserved and code for 141 amino acids. By contrast, the corresponding introns vary in length and show considerable divergence. Comparison of 900 bp of the 5'-flanking region revealed that the X. tropicalis gene contains a conserved proximal 310-bp promoter sequence, comprised of the canonical TATA and CCAAT motifs at homologous positions, and five conserved elements in the same order and at similar positions as previously shown for the corresponding genes of X. laevis. We therefore conclude that these conserved upstream elements may represent regulatory sequences for cell-specific regulation of the adult Xenopus globin genes.

On the nucleus structure and activity of comet 67P/Churyumov-Gerasimenko

Images from the OSIRIS scientific imaging system onboard Rosetta show that the nucleus of 67P/Churyumov-Gerasimenko consists of two lobes connected by a short neck. The nucleus has a bulk density less than half that of water. Activity at a distance from the Sun of >3 astronomical units is predominantly from the neck, where jets have been seen consistently. The nucleus rotates about the principal axis of momentum. The surface morphology suggests that the removal of larger volumes of material, possibly via explosive release of subsurface pressure or via creation of overhangs by sublimation, may be a major mass loss process. The shape raises the question of whether the two lobes represent a contact binary formed 4.5 billion years ago, or a single body where a gap has evolved via mass loss.

Periodic variation in side-chain polarities of T-cell antigenic peptides correlates with their structure and activity.

We present an analysis that synthesizes information on the sequence, structure, and motifs of antigenic peptides, which previously appeared to be in conflict. Fourier analysis of T-cell antigenic peptides indicates a periodic variation in amino acid polarities of 3-3.6 residues per period, suggesting an amphipathic alpha-helical structure. However, the diffraction patterns of major histocompatibility complex (MHC) molecules indicate that their ligands are in an extended non-alpha-helical conformation. We present two mutually consistent structural explanations for the source of the alpha-helical periodicity, based on an observation that the side chains of MHC-bound peptides generally partition with hydrophobic (hydrophilic) side chains pointing into (out of) the cleft. First, an analysis of haplotype-dependent peptide motifs indicates that the locations of their defining residues tend to force a period 3-4 variation in hydrophobicity along the peptide sequence, in a manner consistent with the spacing of pockets in the MHC. Second, recent crystallographic determination of the structure of a peptide bound to a class II MHC molecule reveals an extended but regularly twisted peptide with a rotation angle of about 130 degrees. We show that similar structures with rotation angles of 100-130 degrees are energetically acceptable and also span the length of the MHC cleft. These results provide a sound physical chemical and structural basis for the existence of a haplotype-independent antigenic motif which can be particularly important in limiting the search time for antigenic peptides.