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Oriented films of bacteriophage DNAs of low and intermediate (G + C) content and of different chemical composition were investigated by infrared linear dichroism: PBS1 DNA (28% G + C, uracil replaces thymine), T4 DNA (34% G + C, glucosylated), T4* DNA (34% G + C, unglucosylated), T5 DNA (39% G + C). None of these phage DNAs exhibit a transition to a predominantly pure A form when relative humidity is lowered. At lower relative humidity, the infra- red dichroism of the 1230-cm-1 band changes to mainly parallel orientation with respect to the DNA axis. The pattern of the tansition as a function of relative humidity indicates the presence at low relative humidity of a well-defined form for which the O2- -O3 line of the phosphate group was found to be directed approximately at 52 °C with respect to the helix axis. This allows one to assign the lower-humidity form of these phage DNAs as a predominant C-like form. The comparison of T4 with T4* DNA indicates that glucosylation has no effect on conformational properties of phage DNAs, and particularly on the absence of the B to A structural change. The replacement of the thymine by uracil in DNA of PBS1 has not affected the structural properties of the B form at high humidity nor the absence of the main A form at lower humidity; the transition from the B form to a predominant C-like form was observed.

The effect of superhelical constraint on the structure of covalently closed circular DNA (cccDNA; pBR322) with positive and negative writhe (superturn) has been investigated as a function of decreasing and increasing specific linking difference (mean superhelical density ...). At low and moderate negative superhelical densities ..., the overall average structure is maintained in an unwound B-form slightly modified. The overwound cccDNAs with positive writhe differ from those with negative writhe by an absence of cruciform structure. At high negative densities of supercoiling different changes involving the reversal of twist handedness are shown to lead to the formation of DNA segments in a conformation identical to the left-handed component of form V DNA. [ABSTRACT FROM AUTHOR]

Human pancreatic carboxyl-ester hydrolase is a glycoprotein with a molecular weight of 100 000 and a high content in carbohydrate, 20 %. Sedimentation studies indicate that the molecule resembles an ellipsoid. The results of hydrodynamic and vacuum-ultraviolet circular dichroism investigation allow one to propose a model of the carboxyl-ester hydrolase three-dimensional structure. The enzyme belongs to the 'all β' class of proteins; about 54-60 % of its residues are in β-sheets and in β-turns, most probably forming the surface of an ellipsoid. A similarity with prealbumin structure is proposed and a comparison with structure of other pancreatic enzymes is presented. [ABSTRACT FROM AUTHOR]

Crosslinking induced by ultraviolet light irradiation at 254 nm has been utilized to investigate the structure of chromatin and isolated nucleosomes. The results presented here imply that the four core histones, as well as histone H1, have reactive groups within a bond length of the DNA bases. In nucleosomes depleted of H1, all of the core histones react similarly with the DNA and form crosslinks. In chromatin, the rate of crosslinking of all histones to DNA is essentially similar. Comparison of mononucleosomes, dinucleosomes and whole chromatin shows that the rate of crosslinking increases significantly with increasing number of connected nucleosomes. These differences in the rate of crosslinking are interpreted in terms of interactions between neighbouring nucleosomes on the chromatin fiber, which are absent in an isolated mononucleosome. [ABSTRACT FROM AUTHOR]