Controlling the higher-order structure of DNA-protamine complex

We investigated the interaction of large DNA with protamine, a small basic protein, using fluorescence microscopy and electron microscopy. In most vertebrate sperm cells, a large DNA is packaged by protamine into a highly compact, biologically inactive form of chromatin. The manner of packaging plays a key role in spermatogenesis. We found that, with increase of protamine concentration, DNA molecules show the conformational change from an elongated coil state into a compact state through the occurrence of the intermediate state. We also show that the higher-order structure of DNA-protamine complex can be controlled by optical tweezers (Nd YAG laser, 1064 nm) without any structural modification of DNA such as an attachment of plastic beads. These results are discussed in relation to the biological meaning.