No abstract

We report transient electric dichroism experiments on nucleosomal core particles containing 140 and 175 base pairs of DNA, and on spacerless dinucleosomes. The results indicate that all particles posses a permanent dipole moment. The orientation time of 140 base pair nucleosomes implies an estimated maximum dimension of a = 130 A (a must be at least 111 A), consistent with the disk model. The maximum dimension of the spacerless dinucleosome is estimated to be about 290 A (at least 180 A), ruling out a structure in which two disks are stacked directly on top of each other. The reduced dichroism amplitude indicate that the DNA superhelix axis in nucleosomes aligns perpendicular to the electric field, as expected for a dipole moment directed along a C2 symmetry axis across the disk diameter. Nucleosomes containing 175 base pairs of DNA show a substantially larger dichroism amplitude that do 140 base pair nucleosomes. In the context of the disk model, this result is shown to be consistent with 100 base pairs of DNA per superhelical turn, but not with 80 base pairs per turn.

Staphylococcal nuclease digestion of the complex between DNA and DNA gyrase yields a gyrase-DNA core particle composed of a 140 base pair DNA segment and an active gyrase enzyme. The partial specific volume and S20,w of this purified core complex are measured to be 0.70 cm3/g and 14.5 S, respectively, by sedimentation measurements in H2O and D2O media. The molecular weight of the core complex estimated from equilibrium centrifugation is 470 000; the ratio of the translational frictional coefficient to that of the unsolvated equivalent sphere is calculated to be 1.9. Treatment of free gyrase in solution with dimethyl suberimidate gives three cross-linked species of roughly equal amounts that can be identified as alpha 2, alpha 2 beta, and alpha 2 beta 2. When the gyrase core complex is treated with the same cross-linking agent, 70-80% of the protein is converted to the alpha 2 beta 2 species. These results establish that the gyrase-DNA core complex contains a 140 base pair DNA segment and a tetrameric alpha 2 beta 2 protein.

31P NMR studies of 140 base pair DNA fragments in nucleosomes and free in solution show no detectable change in the internucleotide 31P chemical shift or linewidth when DNA is packaged into nucleosomes. Measurements of 31P spin-lattice relaxation times T1 and 31P-[H] nuclear Overhauser enhancements revealed internal motion with a correlation time of about 4 x 10(-10) sec in double helical DNA, both free in solution and bound to nucleosomal core proteins. This result implies greater dynamic mobility in double helical DNA than has previously been supposed.

A series of dATP and dCTP nucleotide analogs have been synthesized which are modified by attachment of aliphatic linkers containing a functional group to the amino-nitrogen at the hydrogen bonding positions of the bases, that is, at the 6-position of adenine and the 4-position of cytosine. These nucleotides are incorporated into DNA probes by standard nick-translation protocols. DNA probes labeled with biotin derivatives of these nucleotides are effectively hybridized to target DNA sequences and can be detected by a streptavidin and calf intestinal alkaline phosphatase conjugate with a sensitivity (0.25 pg DNA) sufficient for reproducible and rapid detection of single copy genes in a Southern blot of mammalian DNA.Also, a procedure has been developed to allow reprobing of nylon filters that have been hybridized with biotinylated probes and developed with the streptavidin/alkaline phosphatase conjugate and a standard dye system.