A Link between Deoxyribonucleotide Metabolites and Embryonic Cell-Cycle Control.

The egg contains maternal RNAs and proteins, which have instrumental functions in patterning and morphogenesis. Besides these, the egg also contains metabolites, whose developmental functions have been little investigated. For example, the rapid increase of DNA content during the fast embryonic cell cycles poses high demands on the supply of deoxyribonucleotides (dNTPs), which may be synthesized in the embryo or maternally provided [1, 2]. Here, we analyze the role of dNTP in early Drosophila embryos. We found that dNTP levels initially decreased about 2-fold before reaching stable levels at the transition from syncytial to cellular blastoderm. Employing a mutant of the metabolic enzyme serine hydroxymethyl transferase (SHMT), which is impaired in the embryonic synthesis of deoxythymidine triphosphate (dTTP), we found that the maternal supply of dTTP was specifically depleted by interphase 13. SHMT mutants showed persistent S phase, replication stress, and a checkpoint-dependent cell-cycle arrest in NC13, depending on the loss of dTTP. The cell-cycle arrest in SHMT mutants was suppressed by reduced zygotic transcription. Consistent with the requirement of dTTP for cell-cycle progression, increased dNTP levels accelerated the cell cycle in embryos lacking zygotic transcription. We propose a model that both a limiting dNTP supply and interference of zygotic transcription with DNA replication [3] elicit DNA replication stress and checkpoint activation. Our study reveals a specific mechanism of how dNTP metabolites contribute to the embryonic cell-cycle control.