Absence of somatic histone H1 in oocytes and preblastula embryos of Xenopus laevis.

Available data on the occurrence and expression of somatic histone H1 during oogenesis and early embryogenesis of Xenopus laevis are contradictory. In particular the reported presence of a large storage pool of histone H1A in oocytes is difficult to reconcile with the high transcriptional activity of all gene classes in this specific cell type. In the present study we have used polyclonal antibodies raised against somatic Xenopus histone H1 (H1A and H1A/B) for combined immunoblotting experiments to quantitate H1 pools and immunolocalization studies to visualize chromosome-bound H1. Both approaches failed to detect soluble or chromosomal histone H1 in vitellogenic oocytes, eggs, and cleavage-stage embryos up to early blastula. In addition, chromatin assembled in Xenopus egg extract was also negative for histone H1 as revealed by immunofluorescence microscopy. Lampbrush chromosomes not only lacked histone H1 but also the previously identified histone H1-like B4 protein (Smith et al., 1988, Genes Dev. 2, 1284-1295). In contrast, chromosomes of eggs and early embryos fluoresced brightly with anti-B4 antibodies. Our results lend further support to the view that histone H1 expression is developmentally regulated during Xenopus oogenesis and embryogenesis similar to what is known from other species.