Non-coordinated histone synthesis with DNA replication and close temporal association of ribosomal proteins and rRNA syntheses in germinating Vicia faba embryos

Amino acid pools and protein synthesis in germinating maize embryos

Amino acid pools were analyzed in maize (Zea mays L.) axes germinated for 0, 6 and 24 h. Proteins synthesized at early (0-6 h) and late (18-24 h) stages were characterized by gel electrophoresis and fluorography after either (14)C-leucine or (14)C-lysine pulses. An increase of amino acid incorporation after 18-24 h was observed, as well as changes in the protein patterns of the corresponding fluorographs. Analysis of the endogenous amino acid pools showed major changes in contents of proline, alanine, isoleucine, valine, leucine and lysine. A selective increase of lysine incorporation into proteins during the late stage was detected.

Uncoordinated synthesis of histones and DNA by mouse eggs and preimplantation embryos

The temporal coordination of histone and DNA synthesis in mouse eggs and preimplantation embryos were examined. In unfertilized mouse eggs arrested at metaphase II, histone synthesis was observed, but at lower rates than previously reported for fertilized zygotes. Cytosine arabinoside, an inhibitor of DNA synthesis, was used to study the relationship between DNA and histone synthesis during cleavage stages of mouse development. Cytosine arabinoside (50 microM) inhibited by 90% the incorporation of [3H]thymidine into DNA by two-cell embryos and blastocysts. At this concentration, cytosine arabinoside inhibited incorporation of [3H]lysine into blastocyst histones by only 30% and had no effect at the two-cell stage. These results are interpreted to show that DNA synthesis and histone synthesis are not coordinated temporally during early development of the mouse, but may become more so as development proceeds.

Histone synthesis in preimplantation mouse embryos

The synthesis of histones by preimplantation mouse embryos was examined by electrophoresis of [3H]-lysine-labeled acid extracts of cleavage stage embryos. Histones were synthesized by fertilized zygotes, two-cell, eight-cell, and morula stage embryos. Their identity was confirmed by coextraction and coelectrophoresis in a number of systems and differential labeling with [3H]-tryptophan. There was evidence of post-translational modifications of the embryonic histones. The estimated histone synthetic rate was sufficient to provide the chromatin requirements of cleavage division as related to DNA synthesis. The results are discussed in relation to coordination of histone and DNA synthesis in the preimplantation embryo.