Histone synthesis in preimplantation rabbit embryos

Intracellular specific radioactivities of lysine and arginine were measured in preimplantation rabbit embryos. In conjunction with measurements of radioactivity incorporated into total protein and histones and amino acid composition data, absolute rates of synthesis were calculated. Total protein is synthesized at a rate of 0.78 ng/hr/embryo or 0.049 ng/hr/cell for the 2‐day (16‐cell) cleavage embryo and 2,828 ng/hr/embryo or 0.036 ng/hr/cell for the 6‐day (80,000‐cell) blastocyst. Calculated rates of accumulation for histone proteins were 40 pg/hr/embryo for the 2‐day cleavage embryo (5.12% of total protein accumulation) and 96,600 pg/hr/embryo (3.42% of total protein accumulation) for the 6‐day blastocyst. These amounts of histone synthesis are only slightly in excess of that required to package newly synthesized DNA into nucleosomes during the cycle of replication, suggesting that histone and DNA synthesis are quantitatively coordinated. Further, when cytosine arabinoside was used to inhibit DNA synthesis by 97% in cleavage embryos, amino acid incorporation into histone was inhibited by 82%, while total incorporation was reduced by about 20%, indicating that histone and DNA synthesis are also temporally coordinated.