H2A.ZI, A NEW VARIANT HISTONE EXPRESSED DURING XENOPUS EARLY DEVELOPMENT EXHIBITS SEVERAL DISTINCT FEATURES FROM THE CORE HISTONE H2A

We have isolated from a subtractive cDNA library of Xenopus laevis a n ovel transcript, H2A,ZI, which belongs to the H2A,Z variant gene famil y, Characterization of its expression during oogenesis and development shows significant differences from the expression of the core histone H2A, First, H2A,ZI mRNA is mainly detected only during oogenesis and after the midblastula transition, whereas H2A is constitutively expres sed, at much higher levels, throughout embryonic growth, Second, in co ntrast with H2A, the variant H2A.ZI is polyadenylated during developme nt, Third, expression of H2A,ZI is uncoupled from the S phase after ga strula, whereas synthesis of the core histone H2A mRNA is tightly cont rolled to DNA replication, Interestingly, H2A,ZI is less charged in th e N-terminal tail which is crucial for chromatin-mediated repression, The characteristics of H2A,ZI suggest that its incorporation into nucl eosomes would lead to a chromatin structure more competent fbr gene ex pression during development.