Reductive inactivation of digitoxin by Eubacterium lentum cultures

The obligate anaerobe Eubacterium lentum inactivated the cardiac glycoside digitoxin by reducing the double bond in the lactone ring. This conversion was quantitative when the substrate was incubated at a concentration of 10 micrograms/ml. The reduction reaction coincided with the growth phase of the bacterium. The stereochemical configuration at C-20 of the reduction product dihydrodigitoxin was found to be R. Incubation of digitoxigenin and its mono- and bisdigitoxosides individually with E. lentum led to the formation of their respective dihydro derivatives. The configuration at C-20 of these reduced metabolites was also found to be R.     

Imino 1H- and 31P-NMR analysis of the interaction of propidium and ethidium with DNA

At low temperature and low salt concentration, both imino proton and ³¹p-nmr spectra of DNA complexes with the intercalators ethidium and propidium are in the slow-exchange region. Increasing temperature and/or increasing salt concentration results in an increase in the site exchange rate. Ring-current effects from the intercalated phenanthridinium ring of ethidium and propidium cause upfield shifts of the imino protons of A · T and G · C base pairs, which are quite similar for the two intercalators. The limiting induced chemical shifts for propidium and ethidium at saturation of DNA binding sites are approximately 0.9 ppm for A · T and 1.1 ppm for G · C base pairs. The similarity of the shifts for ethidium and propidium, in both the slow- and fast-exchange regions over the entire titration of DNA, shows that a binding model for propidium with neighbor-exclusion binding and negative ligand cooperativity is correct. The fact that a unique chemical shift is obtained for imino protons at intercalated sites over the entire titration and that no unshifted imino proton peaks remain at saturation binding of ethidium and propidium supports a neighbor-exclusion binding model with intercalators bound at alternating sites rather than in clusters on the double helix. Addition of ethidium and propidium to DNA results in downfield shifts in ³¹P-nmr spectra. At saturation ratios of intercalator to DNA base pairs in the titration, a downfield shoulder (approximately ?2.7 ppm) is apparent, which accounts for approximately 15% of the spectral area. The main peak is at ?3.9 to ?4.0 ppm relative to ?4.35 in uncomplexed DNA. The simplest neighbor-binding model predicts a downfield peak with approximately 50% of the spectral area and an upfield peak, near the chemical shift for uncomplexed DNA, with 50% of the area. This is definitely not the case with these intercalators. The observed chemical shifts and areas for the DNA complexes can be explained by models, for example, that involve spreading the intercalation-induced unwinding of the double helix over several base pairs and/or a DNA sequence- and conformation-dependent heterogeneity in intercalation-induced chemical shifts and resulting exchange rates.     

Wrinkled DNA.

The B form of poly d(GC):poly d(GC) in orthorhombic microcrystallites in oriented fibers has a secondary structure in which a dinucleotide is the repeated motif rather than a mononucleotide as in standard, smooth B DNA. One set of nucleotides (probably GpC) has the same conformations as the smooth form but the alternate (CpG) nucleotides have a different conformation at C3'-O3'. This leads to a distinctive change in the orientation of the phosphate groups. Similar perturbations can be detected in other poly d(PuPy):poly d(PuPy) DNAs such as poly d(IC):poly d(IC) and poly d(AT):poly d(AT) in their D forms which have tetragonal crystal environments. This suggests that such perturbations are intrinsic to all stretches of duplex DNA where purines and pyrimidines alternate and may play a role in the detection and exploitation of such sequences by regulatory proteins.     

Conformation of cyclic dipeptides from empirical energy calculations

Empirical energy calculations on cyclo-Gly-X with X- Phe, Tyr, Val, and Leu as a function of the side-chain torsion angles χ indicate that the conformation of minimum energy are characterized by χ₁ = 60°, χ² = 90° for Phe and Try, χ¹ = ?60° for Val and χ¹ = ?60°, χ² = 180° and χ¹ = 60° and χ² = 150° for Leu. The minimum energy conformation of cyclo-Gly-Phe and cyclo-Gly-Val have the side chains of Phe and Val stacked over the poperazinedione ring as suggested by NMR and found for cyclo-Gly-Tyr crystal structure. In contrast, the Leu side chain is expected to exist in an extended or a quasi-folded form.     

Germination ofMusa velutina seeds: Comparison ofin vivo andin vitro systems

Summary In the genusMusa, germination is extremely variable and relatively difficult. Even more difficulties are faced when producing hybrids. The seed yield of hybrids in breeding programs is usually low and often, to ensure the viability and survival of seeds, it is necessary to attempt to germinate a large excess of these seeds. In this context,in vitro embryo culture might be an invaluable tool for obtaining desirable hybrid plants in a short time. Seeds ofMusa velutina were sown in seed trays in a peat-based mixture. Thein vivo seed germination reached 78% but only after 9 mo. Because of this delayed and intermittent germination, embryos were excised from seeds and inoculated onto half-strength Murashige and Skoog (1962) medium, with or without supplementation with various concentrations of gibberellic acid. Light and dark conditions were also used to test their effect on embryo germination. After 2 wk, 82% of embryos germinated in the dark on medium containing 0.1 μM gibberellic acid. Addition of gibberellic acid increased the shoot length and root number over the gibberellic acid-free treatment. Similarly, dark conditions gave a significant increase over light conditions for all the parameters except root number where light or dark conditions did not make any difference. Thus, the present study highlights the importance of various components of thein vitro culture ofMusa embryos and the advantage over direct use of greenhouse-sown seeds both in terms of the time taken to germinate and the final percentage.     

Circumscription of Fulbrightiella gen. nov. and Sherwoodiella gen. nov., Two Novel Genera in the Calotrichaceae (Nostocales,Cyanobacteria)

Three novel strains in Calotrichaceae from tropical habitats were isolated and characterized with regard to their morphology, phylogenetic placement, and secondary structures of conserved domains in the 16S-23S internal transcribed spacer (ITS). The strains fell into two clades formerly identified as Calothrix from freshwater and brackish habitats. Based on both morphology and ecology, they differed from the type species of Calothrix, C. confervicola, which is marine, has wide trichomes with short cells, and narrows abruptly to a hyaline hair. The first clade grouped species with heteropolar filaments widened at the base and narrowed gradually toward the apex but not ending in a hair, with basal heterocytes that are formed in series as the apically placed heterocytes senesce; this clade is being named Fulbrightiella gen. nov., with two named species, F. bharadwajae sp. nov. and F. oahuensis sp. nov. The second clade was comprised of a single species with isopolar trichomes that are untapering as hormogonia, but which widen midfilament and taper toward both ends following growth. These trichomes develop pairs of heterocyte mid-filament, causing fragmentation into heteropolar trichomes with basal heterocytes and ends that taper, but not to a hair. This clade consists of a single species at present, Sherwoodiella mauiensis. With this action, four clades in the Calotrichaceae have been named: Macrochaete, Dulcicalothrix, Fulbrightiella, and Sherwoodiella. Calothrix sensu stricto is truly marine, morphologically distinct, and unsequenced; finding and sequencing the generitype for Calothrix remains as the most important and unfinished task in the revision of the Calotrichaceae.