Multiple modes of DNA compaction by protamine

Abstract

In sperm cells, protamine replaces histones to compact DNA 10-20 times more than in somatic cells. To characterize the extreme compaction, we employed confocal microscopy and optical tweezers to determine the conformations and stability of protamine-bound {lambda}-DNA. Confocal images show increasing compaction of {lambda}-DNA at increasing protamine concentration. In the presence of protamine, single {lambda}-DNA molecules form bends and loops that unravel at 10-40 pN forces as well as coils that shorten the contour length by up to 40% and withstand forces strong enough ([~]55 pN) for strand separation. Strand separation nucleates coils, indicating protamine insertion into DNA bases. Protamine may participate in both local and higher-order chromatin organization, leading to extreme compaction and global transcription silencing. One-Sentence SummaryProtamine compacts sperm DNA in multiple modes, producing bends and loops but also coils that may block transcription.