The relation between 5-amino uracil-induced mitotic synchronization and DNA synthesis

The effect of the pyrimidine analogue 5-amino uracil (5AU) on DNA synthesis in Vicia faba lateral roots has been studied. Earlier reports had suggested that 6 m M 5AU synchronizes nuclear divisions (mitotic index of 40 instead of 10 in controls) in Vicia 12–14 hr after removal from a 24-hr treatment through interference with DNA synthesis at the time of treatment. It was shown in the present study that the mitotic synchronization induced by 5AU can occur while the seedlings are in 5AU, and that even a 2-hr treatment with 12 m M 5AU is sufficient to produce the synchrony. Furthermore, synchronization could not be eliminated by the addition of thymidine to the 5AU, as would have been expected if interference with thymidine synthesis were a cause of synchronization. Autoradiographic studies of 3H-thymidine and 3H-deoxycytidine incorporation into the DNA of the meristematic nuclei of the root tip showed that DNA synthesis goes on in the presence of 4 m M to 6 m M 5AU. The rate of DNA synthesis, however, is slowed down, and S is at least 25–40 per cent longer than in the control. This suggests an extension of S from the normal 7.5 hr to at least 11 hr. After removal of the roots from 4 m M 5AU, the rate of DNA synthesis of the root meristem continues to be slow for about 6 hr and then approaches the control level by 9 hr. The slowing down of DNA synthesis is probably not causally related to the mitotic synchronization. A block in G2 as a possible cause of 5AU-induced mitotic synchronization has been discussed.