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

Lucia Falbo,Simona Fiorani,Ilaria Costa,Dario Parazzoli,Francesca Casagrande,Federica Pezzimenti,Francesco Romeo,Vincenzo Costanzo,Erica Raspelli

doi:10.1038/s41467-020-15180-5

Lucia Falbo, Simona Fiorani + Show 7 more

Open Access

https://doi.org/10.1038/s41467-020-15180-5

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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.

Highlights

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
These observations suggest that inhibitory factors on somatic chromatin prevent DNA replication and that these are removed in unfertilized mitotic eggs and extracts
We found that recombinant SSRP1 alone and in complex with SPT16 pre-incubated with somatic nuclei was able to stimulate their replication in interphase extract without prior incubation in CSF-arrested mitotic egg extracts (Fig. 2a, b)

Summary

Introduction

The number of active replication origins in embryonic nuclei is higher than in somatic ones, ensuring rapid genome duplication during synchronous embryonic cell divisions. 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. The MCM complex remains inactive until Dbf4-dependent kinase (DDK) and cyclin-dependent kinases (CDKs) trigger the recruitment of initiation factors Cdc[45] and GINS forming the CMG complex, which together with DNA polymerases duplicates the DNA1 Factors such as chromatin configuration, DNA sequence and shape and other proteins including histone acetylation enzymes, chromatin remodelers and histone chaperones affect pre-RC assembly. Embryonic replication origin density can be restored on somatic nuclei by incubation in intact unfertilized eggs or their mitotic arrested extracts[6,8,9] This process is accompanied by active chromatin remodeling and removal of somatic chromatin-bound proteins such as transcription factors. Replication origin re-configuration is thought to be essential for nuclear reprogramming obtained through somatic cell nuclear transfer[6,9,10]

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