Unraveling the Histone Replacement Pathway in Sperm

Abstract

In somatic cells, DNA is wrapped around histone octamers in the nucleus. During human spermatogenesis, however,85-90% of the histones are replaced by an arginine-rich protein called protamine. This process allows for efficient packaging of the DNA that creates a more hydrodynamic sperm cell. In this study, we aim to identify the states and pathway for direct histone replacement in fish. Specifically, we are interested in figuring out if replacement happens all at once along the DNA, or if particular histone octamers are replaced before others. To measure replacement, we use a Tethered Particle Motion (TPM) assay. In the TPM assay, a single DNA molecule is tethered to a 0.5-μm-diameter polystyrene particle on one end and to a glass coverslip on the other. Using video microscopy, we track the standard deviation of the motion of the tethered particle, which decreases as the DNA folds. If we see a multistep change in the motion that corresponds to replacement of a single histone octamer, then we will conclude that protamine is replacing histone octamers one at a time. Here, we will present data towards this goal, which is important for understanding how DNA is packaged in sperm.