Abstract
The DNA of most vertebrate sperm cells is packaged by protamines. The primary structure of mammalian protamine I can be divided into three domains, a central DNA binding domain that is arginine-rich and amino- and carboxyl-terminal domains that are rich in cysteine residues. In native bull sperm chromatin, intramolecular disulfide bonds hold the terminal domains of bull protamine folded back onto the central DNA binding domain, whereas intermolecular disulfide bonds between DNA-bound protamines help stabilize the chromatin of mature mammalian sperm cells. Folded bull protamine was used to condense DNA in vitro under various solution conditions. Using transmission electron microscopy and light scattering, we show that bull protamine forms particles with DNA that are morphologically similar to the subunits of native bull sperm chromatin. In addition, the stability provided by intermolecular disulfide bonds formed between bull protamine molecules within in vitro DNA condensates is comparable with that observed for native bull sperm chromatin. The importance of the bull protamine terminal domains in controlling the bull sperm chromatin morphology is indicated by our observation that DNA condensates formed under identical conditions with a fish protamine, which lacks cysteine-rich terminal domains, do not produce as uniform structures as bull protamine. A model is also presented for the bull protamine.DNA complex in native sperm cell chromatin that provides an explanation for the positions of the cysteine residues in bull protamine that form intermolecular disulfide bonds.
Highlights
The formation of intermolecular disulfide bonds between protamines bound to DNA apparently creates a network of cross-linked protamines that accounts for the greater stability of mammalian sperm cell chromatin in comparison with the During vertebrate spermiogenesis, chromatin is dramatically reorganized in developing spermatids as histones and other nonhistone-chromosomal proteins are replaced by arginine-rich oligopeptides known as protamines [1, 2]
Condensation of DNA by Folded Bull Protamine—The principle goal of the present study was to determine whether purified bull protamine (BP) is able to condense DNA in vitro in a manner similar to that observed in native mammalian sperm cell chromatin
Based on volume calculations of the smallest observed condensates, we have concluded that the packing of DNA in vitro by BP is at the same density as DNA in native sperm cell chromatin
Summary
Protamine I is present in all mammalian sperm cells, and its amino acid sequence is relatively conserved among mammals. Protamines condense the DNA of vertebrate sperm cells into thousands of particles that vary in diameter from 50 to 100 sperm cell chromatin of species with protamines that lack cysteine residues (e.g. salmon protamine) [16]. We have sought to determine the role of the amino- and carboxyl-terminal ends of mammalian protamine I These end domains are absent from fish protamines and do not condense DNA as isolated peptide sequences [28]. Croscopy and light-scattering studies of bull protamine-DNA condensates prepared in vitro As part of these investigations, we have condensed DNA with bull protamine in the presence of varying concentrations of salt and the disulfide reducing agent, 2-mercaptoethanol. Bull protamine was found to condense DNA into spherical particles that are within the size range of
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
