A Conserved Element in the Protein-Coding Sequence Is Required for Normal Expression of Replication-Dependent Histone Genes in Developing XenopusEmbryos

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Replication-dependent histone genes in the mouse and Xenopusshare a common regulatory element within the protein-encoding sequence called the CRASα element (coding region activating sequence α) which has been shown to mediate normal expression in vivoand to interact with nuclear factors in vitroin a cell cycle-dependent manner. Thus far, the α element has only been studied in rodent cells in culture, and its effect on histone gene expression during development has not been determined. Here we examine the role of the α element in histone gene expression during Xenopusdevelopment which features a switch in histone gene expression from a replication-independent mode in oocytes to a replication-dependent mode in embryos after midblastula stage. In vivoexpression experiments involving wild-type or α-mutant mouse H3.2 genes show that mutation of the CRASα element results in a fourfold decline of expression in embryos, but does not affect expression in oocytes. Two distinct α sequence-specific binding activities were detected in both oocyte and embryonic extracts. A slowly migrating DNA-binding complex was present at relatively constant levels throughout development from the earliest stages of oogenesis through larval stages. In contrast, levels of a rapidly migrating complex were high in stage I and II oocytes, declined in stage II–VI oocytes, remained low in unfertilized eggs and cleavage stage embryos, and rose dramatically after the midblastula transition. The molecular masses of the factors forming the slow and rapidly migrating complexes were estimated to be approximately 110 and 85 kDa, respectively. DNA-binding activity of the 85 kDa α-binding factor was affected by phosphorylation, binding with higher affinity in the dephosphorylated state. The abrupt increase in DNA-binding activity of the 85-kDa α-binding factor at late blastula coincides with the switch to the replication-dependent mode of histone gene expression. We propose that the conserved α element present in the coding sequence of mouse and Xenopuscore histone genes is required for normal replication-dependent histone expression in the developing Xenopusembryo.