DNA condensation by protamine and arginine-rich peptides: Analysis of toroid stability using single DNA molecules

Both somatic cells and sperm have been shown to take up exogenous DNA, but the frequency of its integration is usually low. Scanning probe microscopy studies of sperm chromatin and synthetic DNA-protamine complexes indicate t hat the coiling of DNA into toroidal subunits, a process initiated in the m aturing spermatid to prepare its genome for delivery into the egg, can be m imicked by simply adding protamine to DNA in vitro. The increased resistanc e of DNA-protamine complexes to nuclease digestion and their structural sim ilarity to native sperm chromatin suggest that the packaging of DNA by prot amine might offer a new approach for improving the efficiency of DNA uptake by sperm. Decondensation experiments performed with individual DNA molecul es have provided a direct measure of the stability of toroids produced usin g salmon protamine and smaller arginine-rich peptides. These experiments sh ow that the arginine content of protamine-related sequences can have a dram atic effect on their rate of dissociation from DNA. This technique and the information it provides can be used to identify protamine analogs that can be bound to DNA to increase the efficiency of its uptake by sperm and other cells. Published 2000 Wiley-Liss, Inc.(dagger)