Primordial germ cell specification requires global transcriptional repression. In C. elegans, the zygote (P0) undergoes four successive asymmetric divisions to generate the germline precursors P1, P2, P3 and finally P4, the germline founder. OMA-1 and OMA-2 (OMA1/2), cytoplasmic proteins degraded after the first mitotic cycle, repress global transcription in P0 and P1 by sequestering TAF-4, an RNA polymerase II pre-initiation complex component, while the maternal protein PIE-1 represses transcript elongation in P2-P4. Now, Rueyling Lin and colleagues report that OMA proteins repress transcription in P2-P4 indirectly by maintaining PIE-1 expression (see ). OMA-1/2, they show, repress zif-1 mRNA translation in oocytes; zif-1 encodes the substrate-binding subunit of the E3 ligase that marks PIE-1 for degradation. MBK-2, a kinase that is activated after fertilisation, controls OMA1/2 function, report the researchers. Thus, they suggest, MBK-2 phosphorylation of OMA1/2 acts as a key developmental switch in the oocyte-to-embryo transition by converting OMA proteins from specific translational repressors in oocytes to global transcriptional repressors in embryos.
