Appearance of two maternally directed histone H2A variants precedes zygotic ubiquitination of H2A in early embryogenesis of Sciara coprophila (Diptera).

A monoclonal antibody raised against Drosophila histone H2A.l (15 kDa) recognizes two H2A isoforms, H2As (16kDa) and H2Af (15 kDa), and a modified ubiquitinated form, uH2A (25 kDa), in the fungus gnat Sciara coprophila. Both variants derive from different mRNAs as shown by in vitro translation, immunoprecipitation, and fluorography. Whereas H2Af is present throughout embryogenesis, the Sciara-specific variant H2As is preferentially expressed during cleavage. These developmental profiles reflect a program of histone H2A expression that is regulated in early embryogenesis by differential translation of maternal mRNAs. Later control is effected by zygotic transcription and degradation of maternal mRNA. Ubiquitination of H2A is first detected at the syncytial blastoderm stage: these results and data obtained in other dipteran systems suggest that this modification event is general for Diptera and shows a tight correlation with initiation of zygotic transcription. The first phases of the histone H2A expression program are maternally controlled and not dependent on fertilization. In sharp contrast, ubiquitination of H2A depends on fertilization-dependent processes rather than on egg activation alone. Expression and modification of histones H2A are therefore differently controlled. The significance of uH2A and H2As is discussed.