The complete sequence of the Bombyx mori fibroin gene has been determined by means of combining a shotgun sequencing strategy with physical map-based sequencing procedures. It consists of two exons (67 and 15 750 bp, respectively) and one intron (971 bp). The fibroin coding sequence presents a spectacular organization, with a highly repetitive and G-rich (approximately 45%) core flanked by non-repetitive 5' and 3' ends. This repetitive core is composed of alternate arrays of 12 repetitive and 11 amorphous domains. The sequences of the amorphous domains are evolutionarily conserved and the repetitive domains differ from each other in length by a variety of tandem repeats of subdomains of approximately 208 bp which are reminiscent of the repetitive nucleosome organization. A typical composition of a subdomain is a cluster of repetitive units, Ua, followed by a cluster of units, Ub, (with a Ua:Ub ratio of 2:1) flanked by conserved boundary elements at the 3' end. Moreover some repeats are also perfectly conserved at the peptide level indicating that the evolutionary pressure is not identical along the sequence. A tentative model for the constitution and evolution of this unusual gene is discussed.

Staphylococcus aureus, a Gram-positive bacterium causes a number of devastating human diseases, such as infective endocarditis, osteomyelitis, septic arthritis and sepsis. S. aureus SraP, a surface-exposed serine-rich repeat glycoprotein (SRRP), is required for the pathogenesis of human infective endocarditis via its ligand-binding region (BR) adhering to human platelets. It remains unclear how SraP interacts with human host. Here we report the 2.05 Å crystal structure of the BR of SraP, revealing an extended rod-like architecture of four discrete modules. The N-terminal legume lectin-like module specifically binds to N-acetylneuraminic acid. The second module adopts a β-grasp fold similar to Ig-binding proteins, whereas the last two tandem repetitive modules resemble eukaryotic cadherins but differ in calcium coordination pattern. Under the conditions tested, small-angle X-ray scattering and molecular dynamic simulation indicated that the three C-terminal modules function as a relatively rigid stem to extend the N-terminal lectin module outwards. Structure-guided mutagenesis analyses, in addition to a recently identified trisaccharide ligand of SraP, enabled us to elucidate that SraP binding to sialylated receptors promotes S. aureus adhesion to and invasion into host epithelial cells. Our findings have thus provided novel structural and functional insights into the SraP-mediated host-pathogen interaction of S. aureus. Staphylococcus aureus is an important pathogen that causes a range of human diseases, such as infective endocarditis, osteomyelitis, septic arthritis and sepsis. The increasing resistance of S. aureus to most of the current antibiotics emphasizes the need to develop new approaches to control staphylococcal infections. As a surface-exposed serine-rich repeat glycoprotein (SRRP), S. aureus SraP is involved in the pathogenesis of infective endocarditis via its ligand-binding region (BR) adhering to human platelets. However, little is known about how SraP interacts with its host receptor(s). Through structural and functional analyses of the BR domain, we have discovered a specific binding of SraP to N-acetylneuraminic acid (Neu5Ac), in agreement with a recent report of the trisaccharide ligand of SraP. Further mutagenesis analysis showed that SraP binding to Neu5Ac and the trisaccharide promotes S. aureus adhesion to and invasion into host epithelial cells. These findings increase our knowledge of surface protein mediated interaction of S. aureus with host epithelial cells.

2-oxogluatarate (2-OG), a metabolite of the highly conserved Krebs cycle, not only plays a critical role in metabolism, but also constitutes a signaling molecule in a variety of organisms ranging from bacteria to plants and animals. In cyanobacteria, the accumulation of 2-OG constitutes the signal of nitrogen starvation and NtcA, a global transcription factor, has been proposed as a putative receptor for 2-OG. Here we present three crystal structures of NtcA from the cyanobacterium Anabaena : the apoform, and two ligand-bound forms in complex with either 2-OG or its analogue 2,2-difluoropentanedioic acid. All structures assemble as homodimers, with each subunit composed of an N-terminal effector-binding domain and a C-terminal DNA-binding domain connected by a long helix (C-helix). The 2-OG binds to the effector-binding domain at a pocket similar to that used by cAMP in catabolite activator protein, but with a different pattern. Comparative structural analysis reveals a putative signal transmission route upon 2-OG binding. A tighter coiled-coil conformation of the two C-helices induced by 2-OG is crucial to maintain the proper distance between the two F-helices for DNA recognition. Whereas catabolite activator protein adopts a transition from off-to-on state upon cAMP binding, our structural analysis explains well why NtcA can bind to DNA even in its apoform, and how 2-OG just enhances the DNA-binding activity of NtcA. These findings provided the structural insights into the function of a global transcription factor regulated by 2-OG, a metabolite standing at a crossroad between carbon and nitrogen metabolisms.