Remodeling of sperm chromatin following fertilization: nucleosome repeat length and histone variant transitions in the absence of DNA synthesis.

Within the first cell cycle following fertilization the average nucleosomal repeat length of sea urchin male pronuclear chromatin declines by 30-40 base pairs to a value typical of that found in the embryo. This decline occurs after a lag of about 30 min postfertilization, and is accompanied by replication of the male chromatin and accumulation of cleavage-stage (CS) core histone variants. When replication is inhibited by greater than 95% with aphidicolin, the decline in repeat length still occurs, although it is slightly retarded. The decline in repeat length also occurs when protein synthesis is blocked by greater than 98% and DNA synthesis by 60-70% with emetine. The adjustment of nucleosome repeat length therefore can occur in vivo without extensive movement of replication forks across the length of the chromatin, or normal progression of the cell cycle, and appears to require no proteins synthesized postfertilization. Blocking of DNA synthesis or protein synthesis also does not prevent the normal histone variant transitions involved in male pronuclear chromatin remodeling. Although their accumulation is slowed, CS core variants eventually become the predominant male pronuclear histones in their classes when replication is inhibited. Since a shortening of the average nucleosomal repeat length of approximately 10-20% is not sufficient to account for this large acquisition of CS variants, some of the sperm (Sp) core histones are probably displaced from the replication-blocked pronucleus. Therefore, accumulation of CS H2A and CS H2B are temporally correlated with the repeat length transition, whereas replication, normal progression of the cell cycle, and the early histone transitions involving SpH1 and SpH2B are not.