PubMed · 3532226
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R Nolden. 1986. [Current improvements and future composites].. https://pubmed.ncbi.nlm.nih.gov/3532226/
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Ultrafast emission and absorption spectroscopies were used to measure the kinetics of DNA-mediated electron transfer reactions between metal complexes intercalated into DNA. In the presence of rhodium(III) acceptor, a substantial fraction of photoexcited donor exhibits fast oxidative quenching (>3 x 10(10) per second). Transient-absorption experiments indicate that, for a series of donors, the majority of back electron transfer is also very fast (approximately 10(10) per second). This rate is independent of the loading of acceptors on the helix, but is sensitive to sequence and pi stacking. The cooperative binding of donor and acceptor is considered unlikely on the basis of structural models and DNA photocleavage studies of binding. These data show that the DNA double helix differs significantly from proteins as a bridge for electron transfer.
Evidence is presented that a compartmentalised protein exists in its native state only within a particular size of aqueous cavity. This behaviour is shown to exist in AOT reverse micelles using fluorescence quenching and circular dichroism (CD) studies of human serum albumin (HSA). In particular, far ultraviolet CD measurements show that a reduction in quencher accessibility to the fluorophore is consistent with the protein being nearest to its native conformation at a waterpool size of around 80 A diameter. We also show that the biexponential fluorescence decay of N-acetyl-L-tryptophanamide (NATA) in AOT reverse micelles arises from the probe being located in two distinct sites within the interfacial region. The more viscous of these two sites is located on the waterpool side of the interface and the other is located on the oil side of the interface.
The mechanism by which gene regulatory proteins gain access to their DNA target sequences in chromatin is not known. We recently showed that nucleosomes are intrinsically dynamic, transiently exposing their DNA to allow sequence-specific protein binding even at buried sites. Here we show that this dynamic behaviour provides a mechanism for cooperativity (synergy) in the binding of two or more proteins to sites on a single nucleosome, even if those proteins do not interact directly with each other in any way. As a consequence of this cooperativity, two proteins binding to the same nucleosome facilitate each other's binding and also control the level of occupancy at each other's sites. This model, with no adjustable parameters, accounts quantitatively for recent reports of cooperative (synergistic) binding to nucleosomes in vitro. We assess the potential importance of this new cooperativity for gene regulation in vivo by comparing its magnitude to free energies of cooperative protein-protein direct contacts having known significance for gene regulation. Possible roles for nucleosome dynamics in eukaryotic gene regulation, and key remaining questions, are discussed.