Phosphorylation of sea urchin histone CS H2A

Phosphorylation of cleavage stage (CS) histones was studied during the first cell cycle in male pronuclei of the sea urchin. Histone CS H2A rapidly incorporated 32PO4 during the replication period, but not before. Peptide mapping and amino acid analysis of radiolabelled CS H2A showed that phosphorylation occurred mainly on serine residues located in the C-terminal region of the molecule. When DNA replication was inhibited with aphidicolin both CS H2A and CS H2B accumulated in male pronuclei at the same rate as in the control culture, whereas accumulation of H3 and H4 histones was reduced. Incorporation of 32PO4 by CS H2A doubled when DNA synthesis was inhibited with aphidicolin. Thus phosphorylation of CS H2A was correlated with transport of CS histones from the egg storage pool to the male pronucleus, but not with chromatin synthesis, indicating that this event precedes nucleosome formation. A role for phosphorylation and dephosphorylation of the CS H2A C-terminal region in modulating transport of stored CS histone dimers and their assembly into nucleosomes is discussed.