LIMITED DNA SYNTHESIS IN THE ABSENCE OF PROTEIN SYNTHESIS IN PHYSARUM POLYCEPHALUM

Abstract

Actidione (cycloheximide), an antibiotic inhibitor of protein synthesis, blocked the incorporation of leucine and lysine during the S phase of Physarum polycephalum. Actidione added during the early prophase period in which mitosis is blocked totally inhibited the initiation of DNA synthesis. Actidione treatment in late prophase, which permitted mitosis in the absence of protein synthesis, permitted initiation of a round of DNA replication making up between 20 and 30 % of the unreplicated nuclear DNA. Actidione treatment during the S phase permitted a round of replication similar to the effect at the beginning of S. The DNA synthesized in the presence of actidione was replicated semiconservatively and was stable through at least the mitosis following antibiotic removal. Experiments in which fluorodeoxyuridine inhibition was followed by thymidine reversal in the presence of actidione suggest that the early rounds of DNA replication must be completed before later rounds are initiated.

REGULATION OF DNA REPLICATION IN THE NUCLEI OF THE SLIME MOLD PHYSARUM POLYCEPHALUM

“…This would be prevented if replication of all parts of the genome, including those which are replicated at a later time of the intermitotic period, were rigidly controlled by a common signal related to mitosis. The finding that G2-phase nuclei, when implanted into S-phase host plasmodia, were not labeled with thymidine-'H even after prolonged exposure to the S-phase environment, and that incorporation of thymidine-'H in these nuclei began immediately after division, would suggest that the beginning of DNA replication, even of those replicons which might start replication at a later time during the intermitotic period (2,4), is controlled by an event which occurs either during or immediately after mitosis. It is possible that the chromosomes are rendered competent for DNA replication by a structural alteration which they undergo as part of the mitotic process.…”

“…The finding that S-phase nuclei continued to incorporate DNA precursors from a G2-phase host FIGURE 4 Smear preparation (fixation, ethanol) from late interphase (40 min before metaphase) host plasmodium which had received postmitotic nuclei. Fixed shortly after incubation with thymidine-3H.…”

Multiple origins of replication in the dihydrofolate reductase amplicons of a methotrexate-resistant chinese hamster cell line.

“…Thus, we have now isolated the equivalent of three early-firing replicons (including their origins) from a well-characterized chromosomal domain. With these tools, it should be possible to determine those properties that are shared by the origins and termini of different replicons and which are therefore likely to be functionally significant.It has been known for several years that there is a temporal order of replication of defined genomic sequences in the DNA synthesis period of higher eucaryotic cells (11,22,29,32,54,56). It is also clear that early-and latereplicating sequences can be interspersed on the same chromosome (11) and that a given sequence can be replicated at different intervals of the S period in different cell types of the same organism (22,29,32).…”

“…It is also clear that early-and latereplicating sequences can be interspersed on the same chromosome (11) and that a given sequence can be replicated at different intervals of the S period in different cell types of the same organism (22,29,32). In fiber autoradiographic studies, mammalian chromosomal DNA has been shown to be synthesized through the agency of thousands of tandemly arranged replication units (replicons; 31).…”

Reassociation Kinetics of Nuclear DNA from Physarum polycephalum

“…We followed DNA synthesis by thimidine incorporation [18,19] €or each consecutive quarter of S phase and determined renaturation kinetics by hydroxyapatite chromatography. I n the second half of S phase (Fig.2B) we observed a pattern very similar to bulk DNA (Fig.…”