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<jats:p>Cichlid fish species of Lake Victoria can interbreed without loss of fertility but are sexually isolated by mate choice. Mate choice is determined on the basis of coloration, and strong assortative mating can quickly lead to sexual isolation of color morphs. Dull fish coloration, few color morphs, and low species diversity are found in areas that have become turbid as a result of recent eutrophication. By constraining color vision, turbidity interferes with mate choice, relaxes sexual selection, and blocks the mechanism of reproductive isolation. In this way, human activities that increase turbidity destroy both the mechanism of diversification and that which maintains diversity.</jats:p>

<jats:p> The crystal structure of squalene-hopene cyclase from <jats:italic>Alicyclobacillus acidocaldarius</jats:italic> was determined at 2.9 angstrom resolution. The mechanism and sequence of this cyclase are closely related to those of 2,3-oxidosqualene cyclases that catalyze the cyclization step in cholesterol biosynthesis. The structure reveals a membrane protein with membrane-binding characteristics similar to those of prostaglandin-H <jats:sub>2</jats:sub> synthase, the only other reported protein of this type. The active site of the enzyme is located in a large central cavity that is of suitable size to bind squalene in its required conformation and that is lined by aromatic residues. The structure supports a mechanism in which the acid starting the reaction by protonating a carbon-carbon double bond is an aspartate that is coupled to a histidine. Numerous surface α helices are connected by characteristic QW-motifs (Q is glutamine and W is tryptophan) that tighten the protein structure, possibly for absorbing the reaction energy without structural damage. </jats:p>

<jats:p>Terpene cyclases catalyze the synthesis of cyclic terpenes with 10-, 15-, and 20-carbon acyclic isoprenoid diphosphates as substrates. Plants have been a source of these natural products by providing a homologous set of terpene synthases. The crystal structures of 5-epi-aristolochene synthase, a sesquiterpene cyclase from tobacco, alone and complexed separately with two farnesyl diphosphate analogs were analyzed. These structures reveal an unexpected enzymatic mechanism for the synthesis of the bicyclic product, 5-epi-aristolochene, and provide a basis for understanding the stereochemical selectivity displayed by other cyclases in the biosynthesis of pharmacologically important cyclic terpenes. As such, these structures provide templates for the engineering of novel terpene cyclases.</jats:p>

<jats:p>The crystal structure of pentalenene synthase at 2.6 angstrom resolution reveals critical active site features responsible for the cyclization of farnesyl diphosphate into the tricyclic hydrocarbon pentalenene. Metal-triggered substrate ionization initiates catalysis, and the α-barrel active site serves as a template to channel and stabilize the conformations of reactive carbocation intermediates through a complex cyclization cascade. The core active site structure of the enzyme may be preserved among the greater family of terpenoid synthases, possibly implying divergence from a common ancestral synthase to satisfy biological requirements for increasingly diverse natural products.</jats:p>

<jats:p>Structure determination at 2.4 angstrom resolution shows that λ-exonuclease consists of three subunits that form a toroid. The central channel is funnel shaped, tapering from an inner diameter of about 30 angstroms at the wider end to 15 angstroms at the narrow end. This is adequate to accommodate the DNA substrate and thus provides a structural basis for the ability of the enzyme to sequentially hydrolyze thousands of nucleotides in a highly processive manner. The results also suggest the locations of the active sites and the constraints that limit cleavage to a single strand.</jats:p>

<jats:p> Renal 25–hydroxyvitamin D <jats:sub>3</jats:sub> 1α-hydroxylase [1α(OH)ase] catalyzes metabolic activation of 25–hydroxyvitamin D <jats:sub>3</jats:sub> into 1α,25–dihydroxyvitamin D <jats:sub>3</jats:sub> [1α,25(OH) <jats:sub>2</jats:sub> D <jats:sub>3</jats:sub> ], an active form of vitamin D, and is inhibited by 1α,25(OH) <jats:sub>2</jats:sub> D <jats:sub>3</jats:sub> . 1α(OH)ase, which was cloned from the kidney of mice lacking the vitamin D receptor (VDR <jats:sup>−</jats:sup> <jats:sup>/</jats:sup> <jats:sup>−</jats:sup> mice), is a member of the P450 family of enzymes (P450 <jats:sub>VD1</jats:sub> <jats:sub>α</jats:sub> ). Expression of 1α(OH)ase was suppressed by 1α,25(OH) <jats:sub>2</jats:sub> D <jats:sub>3</jats:sub> in VDR <jats:sup>+/+</jats:sup> and VDR <jats:sup>+/</jats:sup> <jats:sup>−</jats:sup> mice but not in VDR <jats:sup>−</jats:sup> <jats:sup>/</jats:sup> <jats:sup>−</jats:sup> mice. These results indicate that the negative feedback regulation of active vitamin D synthesis is mediated by 1α(OH)ase through liganded VDR. </jats:p>

<jats:p>To adhere to solid surfaces, marine mussels produce byssal threads, each of which is a stiff tether at one end and a shock absorber with 160 percent extensibility at the other end. The elastic extensibility of proximal byssus is extraordinary given its construction of collagen and the limited extension (less than 10 percent) of most collagenous materials. From the complementary DNA, we deduced that the primary structure of a collagenous protein (preCol-P) predominating in the extensible proximal portion of the threads encodes an unprecedented natural block copolymer with three major domain types: a central collagen domain, flanking elastic domains, and histidine-rich terminal domains. The elastic domains have sequence motifs that strongly resemble those of elastin and the amorphous glycine-rich regions of spider silk fibroins. Byssal thread extensibility may be imparted by the elastic domains of preCol-P.</jats:p>