Major transitions in histone gene expression do not occur during development in Xenopus laevis

In light of the parallels that exist in the structure of histone genes in sea urchins and in the frog, Xenopus laevis, and in the early development of these animals, it has been thought that Xenopus histone gene expression might be subject to the type of developmental regulation observed in sea urchins. We have examined the patterns of histone mRNA accumulation in Xenopus oocytes and embryos by primer extension and S1 nuclease protection techniques. The data demonstrate that histone genes which are active in Xenopus oocytes, and which contribute to large pools of histone mRNA in the absence of DNA replication, are also transcriptionally active in late embryos and in cultured cells. These results suggest that, rather than activating distinct sets of histone genes at different developmental stages, the developing frog embryo reprograms the expression of histone genes active in nondividing oocytes so that their expression becomes coupled to DNA replication subsequently during embryogenesis.