Protamine folds DNA into flowers and loop stacks

Cationic Residues of the HIV-1 Nucleocapsid Protein Enable DNA Condensation to Maintain Viral Core Particle Stability during Reverse Transcription

The HIV-1 nucleocapsid protein (NC) is a multifunctional viral protein necessary for HIV-1 replication. Recent studies have demonstrated that reverse transcription (RT) completes in the intact viral capsid, and the timing of RT and uncoating are correlated. How the small viral core stably contains the ~10 kbp double stranded (ds) DNA product of RT, and the role of NC in this process, are not well understood. We showed previously that NC binds and saturates dsDNA in a non-specific electrostatic binding mode that triggers uniform DNA self-attraction, condensing dsDNA into a tight globule against extending forces up to 10 pN. In this study, we use optical tweezers and atomic force microscopy to characterize the role of NC's basic residues in dsDNA condensation. Basic residue mutations of NC lead to defective interaction with the dsDNA substrate, with the constant force plateau condensation observed with wild-type (WT) NC missing or diminished. These results suggest that NC's high positive charge is essential to its dsDNA condensing activity, and electrostatic interactions involving NC's basic residues are responsible in large part for the conformation, size, and stability of the dsDNA-protein complex inside the viral core. We observe DNA re-solubilization and charge reversal in the presence of excess NC, consistent with the electrostatic nature of NC-induced DNA condensation. Previous studies of HIV-1 replication in the presence of the same cationic residue mutations in NC showed significant defects in both single- and multiple-round viral infectivity. Although NC participates in many stages of viral replication, our results are consistent with the hypothesis that cationic residue mutations inhibit genomic DNA condensation, resulting in increased premature capsid uncoating and contributing to viral replication defects.

Multiple modes of DNA compaction by protamine

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 λ-DNA. Confocal images show increasing compaction of λ-DNA at increasing protamine concentration. In the presence of protamine, single λ-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.

Effects of histones related to sperm chromatin on embryo development and ART outcomes

In the process of spermatogenesis and maturation, histones of the sperm nucleus were gradually replaced by protamine. Abnormal sperm nucleoprotein histotype conversion can make sperm DNA unstable and affect sperm function. The aim of this study is to investigate the impact of high and low proportion of sperm histone positivity in semen sample on embryonic development and assisted reproductive technology results, and to evaluate its diagnostic value in assisted reproduction. Sperm nuclear status was detected with aniline blue staining. Under acidic conditions, aniline blue combines with histones rich in lysine residues to form blue compounds. The groups were divided according to the critical value of sperm histone positive ratio of 30%. Using the intracytoplasmic sperm injection procedure, the fertilization rate and normal fertilization rate in the normal sperm histone positive ratio group were significantly higher than those in the abnormal group, and the difference was statistically significant (P < .001). Using the in vitro fertilization procedure, the effect of sperm histone positive ratio on each index was not statistically different. Overall the study provides some preliminary evidence that abnormal sperm histones may be a factor that affects the fertilization success of intracytoplasmic sperm injection procedures. However, more research is needed to confirm this finding to determine the exact mechanism by which abnormal sperm histones affect fertilization.