SSRP1-mediated histone H1 eviction promotes replication origin assembly and accelerated development

Abstract

In several metazoans, the number of active replication origins in embryonic nuclei is higher than in somatic ones, ensuring rapid genome duplication during synchronous embryonic cell divisions. High replication origin density can be restored by somatic nuclear reprogramming. However, mechanisms underlying high replication origin density formation coupled to rapid cell cycles are poorly understood. Here, using Xenopus laevis, we show that SSRP1 stimulates replication origin assembly on somatic chromatin by promoting eviction of histone H1 through its N-terminal domain. Histone H1 removal derepresses ORC and MCM chromatin binding, allowing efficient replication origin assembly. SSRP1 protein decays at mid-blastula transition (MBT) when asynchronous somatic cell cycles start. Increasing levels of SSRP1 delay MBT and, surprisingly, accelerate post-MBT cell cycle speed and embryo development. These findings identify a major epigenetic mechanism regulating DNA replication and directly linking replication origin assembly, cell cycle duration and embryo development in vertebrates.

Epigenetic regulation of replication origin assembly: A role for histone H1 and chromatin remodeling factors

“…[12] Overall these changes might be promoted by increased nucleosome mobility along the DNA induced by the loss of histone H1 and might be independent of direct effects of SSRP1 and the FACT complex on core histone association to DNA as no changes were found in histone H2A, H2B, H3, and H4 binding to chromatin by overexpressing SSRP1 in Xenopus. [12] Significantly, the N-terminus domain of SSRP1 was shown to bind H1 with high affinity and, surprisingly, was demonstrated to be necessary and sufficient to mediate SSRP1 effects, which were independent of SPT16 subunit. Recent work has suggested that SSRP1 can form homodimers in vitro that bind H3-H4 histones through the same PH2 domain involved in the formation of heterodimers with SPT16.…”

“…Consistent with this, histone H1 binding to sperm chromatin directly inhibits ORC and MCM chromatin loading. [12] The mechanism by which histone H1 directly prevents ORC binding remains to be established and could be related to histone H1-mediated inhibition of chromatin remodeling, [70] competition with other chromatin-binding proteins [71] or nucleosomes stabilization. [71,72] Importantly, these inhibitory actions of H1 can be reversed, as demonstrated by recent experiments involving SSRP1, which together with SPT16 forms the chromatin remodeling FACT complex.…”

“…[71,72] Importantly, these inhibitory actions of H1 can be reversed, as demonstrated by recent experiments involving SSRP1, which together with SPT16 forms the chromatin remodeling FACT complex. [12]…”

“…[129] This hypothesis could be tested in systems which are able to support both DNA replication and transcription in vitro such as embryo extracts. [12] CONCLUSIONS AND OUTLOOK…”

“…[ 8 ] A similar redistribution is achieved when nuclei are incubated in interphase egg extracts at low density. [ 12 ] These observations suggest that meiotic‐arrested and interphase extracts contain factors capable of removing epigenetic constraints present on somatic nuclei preventing replication origin assembly.…”