Complementary roles of human protamine 1 and spermine in giant DNA compaction

During sperm maturation, protamines progressively replace histones and compact the paternal genome. Spermine (SPM), a naturally occurring tetravalent polyamine that induces DNA compaction, is also abundant in seminal fluid. However, how SPM and protamines jointly influence higher-order structural transitions of giant DNA remains poorly understood. Here, we investigated the cooperative effects of human protamine 1 (PRM1) and SPM using single-molecule fluorescence microscopy. PRM1 induced a continuous conformational transition from coil to globule through intermediate conformations, whereas SPM induced a discrete coil-globule transition. Notably, under coexistence conditions with low concentrations of PRM1, SPM promoted structural progression along the PRM1-induced continuous folding pathway toward the fully compact globule state. Quantitative analysis supported these observations and demonstrated distinct yet cooperative modes of DNA compaction by PRM1 and SPM. These findings demonstrate complementary roles of human PRM1 and SPM in giant DNA compaction and provide a physicochemical framework for understanding genome compaction during sperm maturation.