Theory of DNA melting curves

Exact algorithms for the calculation of melting curves of heterogeneous DNA with N base pairs apparently require computer time proportional to N². However, it is shown that a decomposition of the loop entropy factor into a sum of I exponential functions (1) gives an extremely accurate approximation to the loop entropy factor for small values of I and (2) makes the computer time for the exact algorithms proportional to I·N. In effect, exact results for melting curves and lengths of helix or coil stretches are obtained with computer time comparable to that required for the Frank-Kamenetskii approximation. The remarkable accuracy of the latter for the fraction of helical content (errors of 0.01–0.05) is confirmed, but appreciably larger errors are found for the lengths of helix or coil stretches (typical errors of 30–100%).     

The molecular organization of the beta-globin complex of the deer mouse, Peromyscus maniculatus

Recombinant DNA clones have been isolated that contain 80 kb of the beta-globin complex from the deer mouse, Peromyscus maniculatus. Comparisons of this complex with that from the laboratory mouse, Mus domesticus (with an order 5'-Hbby, Hbb-bhO, Hbb-bhl, Hbb-bh2, Hbb-bh3, Hbb-bl, Hbb-b2 3') highlight organizational trends in the beta-globin complex since the two species diverged. Unlike other mammals studied thus far, the deer mouse possesses three adult genes. Partial sequence analysis indicates that each of the three adult genes is intact and hence may be functional. Hybridization of one of the two Mus pseudogenes, Hbb-bh3, to genomic blots from Peromyscus reveals that it has a homologous counterpart in Peromyscus. Homologous genes to the two gamma-like Mus genes, Hbb-bhO and Hbb-bhl, are also found in Peromyscus. The strong hybridization between the Hbb-bhl genes and significant nucleotide similarity between the Hbb-bhO genes suggest that both pairs are important for the ontogeny of these mice although no known product has been identified for the Hbb-bhO genes. The presence of Hbb-bhO and Hbb-bhl in Peromyscus suggests that the duplication that created this related gene set occurred before the two lineages diverged. A single gene for Hbb-y has been isolated from Peromyscus. The adult region in Peromyscus has undergone significant divergence from the same region in Mus, having three rather than two adult genes, the acquisition of at least 15 kb of extra DNA relative to Mus, and possibly the loss of the Hbb-bh2 pseudogene. The nonadult region of the complex, in contrast, contains the same set of genes apparently distributed over the same amount of DNA as in the Mus beta- globin complex. This observation suggests that the embryonic region of the complex is more evolutionarily stable than the adult region.      

25S-Dihydrodracogenin: A new spirostan sapogenin from Semele androgyna

From stems and leaves of Semele androgyna L. the new steroid sapogenin 25S-dihydrodracogenin (Ia) has been isolated and its structure confirmed by conversion into the acetates of 25R-dihydrosansevierigenin (Id) and ruscogenin (If) and by partial hydrogenation of dracogenin.     

Calorimetric determination of cooperative interactions in high affinity binding processes

It is demonstrated that isothermal titration calorimetry can be used to determine cooperative interaction energetics even for extremely tight binding processes in which the binding affinity constants are beyond the limits of experimental determination. The approach is based on the capability of calorimetry to measure the apparent binding enthalpy at any degree of ligand saturation. When calorimetric measurements are performed under conditions of total association at partial saturation, the dependence of the apparent binding enthalpy on the degree of saturation is a function only of the cooperative binding interactions. The method developed in this paper allows an independent estimation of cooperative energetic parameters without the need to simultaneously estimate or precisely know the value of the association constants. Since total ligand association at partial saturation is achieved only at macromolecular concentrations much larger than the dissociation constants, the method is especially suited for high and very high affinity processes. Biological associations in this category include fundamental cellular processes like cell surface receptor binding or protein-DNA interactions.