DNA-Protamine Toroids Pull on their Attachment Points

Abstract

The fundamental unit of sperm chromatin is the DNA-protamine toroid which contains 50 kbp of DNA and is shaped like a donut with an outer diameter of 100 nm. This unique form of chromatin occurs when histones are displaced from nucleosomes and replaced by protamines during the differentiation of the sperm cell, known as spermiogenesis. This condenses the sperm genome into a much smaller volume, at the physical limit of packing, to protect it from exogenous insult and inactivate it during its journey to the egg. To determine if biomechanical forces participate in the formation of toroids we decided to examine this process at the single molecule level. Using dual optical traps to expose a single lambda phage DNA molecule to mammalian protamine in a multichannel flow cell we were able to show that toroids exert significant forces on their attachment points. Elasticity measurements of the toroids revealed that they are relatively inflexible. We will discuss what these measurements imply about the architecture of chromatin within the mature sperm cell.