Synthesis and assembly of self-complementary calix[4]arenes.

A calix[4]arene was designed to reversibly dimerize and form an egg-shaped enclosure. Adhesive interactions in the assembly were provided by four self-associating ureas, which form a cyclic array containing 16 hydrogen bonds. The synthesis was completed in four steps from the previously described O,O',O",O"'-tetrabenzylcalix[4]arene. Evidence for dimerization of the calixarene tetraurea was provided by H NMR, mass spectrometry, and the observation of encapsulated molecules. The resulting cavity was of sufficient size to capture guests such as ethyl benzene and p-xylene.

Polycaps: Reversibly formed polymeric capsules

Described are assemblies consisting of polymeric capsules, “polycaps,” formed from two calix[4]arene tetraureas covalently connected at their lower rims. In these structures self-assembly leads to reversibly formed capsule sites along a chain, reminiscent of beads on a string. Their dynamic behavior is characterized by 1H NMR spectroscopy through encapsulation of guest species, reversible polymerization, and the formation of sharply defined hybrid capsules.

Characterization of the Saccharomyces cerevisiae ERG26 gene encoding the C-3 sterol dehydrogenase (C-4 decarboxylase) involved in sterol biosynthesis

All but two genes involved in the ergosterol biosynthetic pathway in Saccharomyces cerevisiae have been cloned, and their corresponding mutants have been described. The remaining genes encode the C-3 sterol dehydrogenase (C-4 decarboxylase) and the 3-keto sterol reductase and in concert with the C-4 sterol methyloxidase (ERG25) catalyze the sequential removal of the two methyl groups at the sterol C-4 position. The protein sequence of the Nocardia sp NAD(P)-dependent cholesterol dehydrogenase responsible for the conversion of cholesterol to its 3-keto derivative shows 30% similarity to a 329-aa Saccharomyces ORF, YGL001c, suggesting a possible role of YGL001c in sterol decarboxylation. The disruption of the YGL001c ORF was made in a diploid strain, and the segregants were plated onto sterol supplemented media under anaerobic growth conditions. Segregants containing the YGL001c disruption were not viable after transfer to fresh, sterol-supplemented media. However, one segregant was able to grow, and genetic analysis indicated that it contained a hem3 mutation. The YGL001c (ERG26) disruption also was viable in a hem 1Δ strain grown in the presence of ergosterol. Introduction of the erg26 mutation into an erg1 (squalene epoxidase) strain also was viable in ergosterol-supplemented media. We demonstrated that erg26 mutants grown on various sterol and heme-supplemented media accumulate nonesterified carboxylic acid sterols such as 4β,14α-dimethyl-4α-carboxy-cholesta-8,24-dien-3β-ol and 4β-methyl-4α-carboxy-cholesta-8,24-dien-3β-ol, the predicted substrates for the C-3 sterol dehydrogenase. Accumulation of these sterol molecules in a heme-competent erg26 strain results in an accumulation of toxic-oxygenated sterol intermediates that prevent growth, even in the presence of exogenously added sterol.

Functional specificity of Hoxa-4 in vertebral patterning lies outside of the homeodomain.

The Hox family of proteins plays a central role in establishing the body plan of a wide range of metazoan organisms. Each member of this family of transcriptional regulators has a distinct functional specificity, yet they bind to similar DNA target sequences through their conserved homeodomain. The mechanisms whereby Hox proteins achieve their diverse specificities in vivo remain undefined. Using the opposing effects of Hoxa-4 and Hoxc-8 in vertebral patterning, we demonstrate by replacing the homeodomain of Hoxa-4 with that of Hoxc-8 that the functional specificity of Hoxa-4 does not track with the homeodomain. These observations provide evidence that other regions of Hox proteins play an important role in mediating functional specificity during mammalian embryogenesis.