DNA is typically packaged by histones into chromatin, enabling both structural organization and gene regulation. However, during spermiogenesis, this chromatin structure undergoes a dramatic reorganization. Small arginine-rich protamine proteins replace histones, compacting the DNA by a factor of ∼10. This ensures proper sperm morphology and motility and is critical for healthy sperm development. Although the structural role of protamines in mature sperm has been established, the process by which protamine replaces histones is poorly understood. One possible model is an active displacement model whereby protamine and histones compete to bind to the DNA. In contrast, another possible model is the electrostatic screening model whereby Debye screening by the protamine would cause unbinding of the histones from the DNA. In the end, both models should result in the formation of a looped DNA saturated in protamine; however, in the electrostatic screening model, the DNA unfolds. We are testing these models using a partial digestion assay to look for unfolding. If the electrostatic screening model is correct, we expect to see digested DNA. If the active displacement model occurs, we should see intact DNA. We will also compare our results to data taken with atomic force microscopy (AFM) and tethered particle motion (TPM). Our work will present experimental progress toward distinguishing between these models and understanding how protamine directly replaces histones during spermiogenesis.
Nwaigbo et al. (2026) studied this question.