Phosphorylation of histone H3 is required for proper chromosome condensation and segregation

Phosphorylation of histone H3 at serine 10 occurs during mitosis in diverse eukaryotes and correlates closely with mitotic and meiotic chromosome cond ensation. To better understand the function of H3 phosphorylation in vivo, we created strains of Tetrahymena in which a mutant H3 gene (S10A) was the only gene encoding the major H3 protein. Although both micronuclei and macr onuclei contain H3 in typical nucleosomal structures, defects in nuclear di visions were restricted to mitotically dividing micronuclei; macronuclei, w hich are amitotic, showed no defects. Strains lacking phosphorylated H3 sho wed abnormal chromosome segregation, resulting in extensive chromosome loss during mitosis. During meiosis, micronuclei underwent abnormal chromosome condensation and failed to faithfully transmit chromosomes. These results d emonstrate that H3 serine 10 phosphorylation is causally linked to chromoso me condensation and segregation in vivo and is required for proper chromoso me dynamics.