Rapid pronucleus assembly using cytoplasmic RNAs in fertilized eggs of <i>Xenopus laevis</i>

The small hypercondensed sperm nucleus undergoes a dramatic transformation into a large, round pronucleus with relaxed chromatin during the brief cleavage period in metazoan embryos, enabling the activation of chromatin functions necessary for subsequent development. However, it remains unclear whether the egg cytoplasm-specific physicochemical properties play a role in pronucleus assembly. Here, we evaluated the impact of abundant RNAs in eggs on pronucleus assembly in the <i>Xenopus laevis</i> cell-free reconstitution system. We found that the introduction of RNAs at an appropriate concentration led to a rapid nuclear growth, more dispersed chromatin distribution, and dissociation of sperm-specific nuclear proteins from the chromatin. These chromatin remodeling properties, which were reproducible through the introduction of negatively charged compounds, facilitated the incorporation of somatic histones into chromatin in the reconstituted nuclei. Based on these findings, we propose that the remodeled chromatin by negatively charged cytoplasmic RNAs accelerates rapid decondensation of negatively charged chromatin and pronucleus assembly during the brief cleavage period following fertilization.