Studies on histone phosphorylation during transitions in chromatin str
ucture occurring in vivo during spermatogenesis and early embryogenesi
s in sea urchins are reviewed and evaluated in the light of recent stu
dies on histone phosphorylation occurring during chromatin synthesis i
n frog egg extracts in vitro and evidence that protein kinases and pho
sphatases play direct roles in the regulation of cellular structure. S
perm-specific histone variants Sp H1 and Sp H2B are maintained as phos
phorylated derivatives N and O/P throughout spermatogenesis and early
embryogenesis and egg specific histone variants CS H1 and CS H2A are p
hosphorylated during early embryogenesis. These developmental correlat
ions provide clues about the roles of histone phosphorylation in contr
ol of chromatin structure in vivo and provide a basis for the interpre
tation of data obtained from in-vitro sperm chromatin remodeling in eg
g extracts and from biochemical studies on the effects of histone phos
phorylation on DNA binding. The potential consequences for chromatin s
tructure of the various histone phosphorylation events observed in sea
urchins and frog egg extracts are discussed.