FIG. 5.

Histone exchange occurs in the Xenopus oocyte endogenous lampbrush chromosomes. Oocytes were injected with mRNA encoding Flag-H3 and processed for immunofluorescence immediately (0 h) and after 1, 2, 4, and 6 h. Representative images of chromosomes were recorded by phase contrast and immunofluorescence for Flag-H3 (FITC) and DNA (DAPI). Merged images are colored green for Flag-H3 and red for DNA: the overlap of signal is orange/yellow. (A) Only a few labeled foci were seen over the chromomeres (Cm) immediately after injection (0 h), whereas incorporation of Flag-H3 became more extensive over chromomeres by 2 h and extended into lateral chromatin loops (LL) by 4 h following injection. Incorporation was restricted to the chromosomes (Ch) and was not seen in nucleoli (No) or Cajal bodies (CB). (B) Higher magnification showing distinct foci of incorporation of Flag-H3 at 1 h over and adjacent to chromomeres. Immunofluorescence is located at sites in register in homologous chromosomes (white lines). (C) Higher magnification of chromosome arms in a preparation made at 6 h after injection showing punctate incorporation of Flag-H3 around the axes of lateral loops and no incorporation in many chromomeres or in telomeres (Tel).