Poly(ADP‐ribosylation) of atypical CS histone variants is required for the progression of S phase in early embryos of sea urchins

The patterns of poly(ADP‐ribosylation) in vivo of CS (cleavage stage) histone variants were compared in sea urchin zygotes at the entrance and the exit of S1 and S2 in the initial developmental cell cycles. This post‐translational modification was detected by Western immunoblots with rabbit sera anti‐poly(ADP‐ribose) that was principally reactive against ADP‐ribose polymers and slightly against ADP‐ribose oligomers. The effect of 3 aminobenzamide (3‐ABA), an inhibitor of the poly(ADP‐ribose) synthetase, on S phase progression was determined in vivo by measuring the incorporation of ³H thymidine into DNA. The results obtained indicate that the CS histone variants are poly(ADP‐ribosylated) in a cell cycle dependent manner. A significantly positive reaction of several CS variants with sera anti‐poly(ADP‐ribose) was found at the entrance into S phase, which decreases after its completion. The incubation of zygotes in 3‐ABA inhibited the poly(ADP‐ribosylation) of CS variants and prevented both the progression of the first S phase and the first cleavage division. These observations suggest that the poly(ADP‐ribosylation) of atypical CS histone variants is relevant for initiation of sea urchin development and is required for embryonic DNA replication.