Shortest nucleosomal repeat lengths during sea urchin development are found in two-cell embryos.

ABSTRACT

Prior to fertilization, sperm possess one of the longest nucleosome repeat lengths yet determined [approximately 250 base pairs (bp) for the sea urchin Strongylocentrotus purpuratus]. We show here that the two-cell embryo has an average repeat size of 189 +/- 2 bp as probed by micrococcal nuclease; this is the shortest average nucleosomal subunit reported for S. purpuratus. By the eight-cell stage, the average nucleosome repeat increases to 201 +/- 2 bp, and it subsequently increases further during development. These results indicate that a dramatic rearrangement of chromatin occurs upon fertilization and that this chromatin remodeling continues through early development. When two-cell embryos are labeled for 30 min with [3H]thymidine and digested briefly, they exhibit nuclease-hypersensitive fragments averaging 308 bp in size, which are consistent with the size of protected DNA units in replication intermediate complexes at blastula stage (as described by Levy and Jacob [Levy, A., & Jacob, K. M. (1978) Cell (Cambridge, Mass.) 14, 259]). Our results are consistent with two general propositions (1) long repeat lengths are found in highly differentiated cells, and (2) short repeat lengths are characteristic of cells more active in cell division. Our data would also imply that a rapid increase in the DNA complement, e.g., in the transition from haploid to diploid state following fertilization, is accompanied by a shortening of the average size of DNA in a nucleosome after replication.