title: In the higher plant Pisum sativum maturation of nascent DNA is blocked by cycloheximide, but only after 4-8 replicons are joined.
abstract: Velocity sedimentation in alkaline sucrose gradients, single cell autoradiography and cytophotometry were used to determine if protein synthesis is required for the maturation of nascent replicons to chromosomal-sized molecules in cultured pea-root cells. The results obtained showed that cycloheximide at 5 and 10 microgram/ml, added either before or during labeling with tritiated thymidine, blocked maturation of nascent DNA at an intermediate size of 72-140 X 10(6) daltons single-stranded DNA. To reach this size, nascent replicons - which are 18 X 10(6) daltons single-stranded DNA each - were replicated and groups of 4-8 replicons were joined even though protein synthesis was reduced to 15% of the control. Further maturation of the nascent molecules to chromosomal size, however, was prevented and this resulted in the accumulation of nascent molecules in the 72-140 X 10(6) daltons range. The experiments also showed that the joining of nascent replicons is not an absolute function of late S or G2 phase of the cell cycle, since cells treated with cycloheximide and blocked in mid-S phase had nascent DNA of a size corresponding to 4-8 joined replicons. Finally, the results support the hypothesis that at least one step in the process of nascent DNA maturation may require replication, during late-S phase, of DNA segments that are interspersed within replicon-clusters that replicate early in the S phase.
mesh_terms: D002467:Cell Nucleus / Q000378:metabolism; D003513:Cycloheximide / Q000494:pharmacology*; D004247:DNA / Q000235:genetics*; D004261:DNA Replication / Q000187:drug effects*; D007700:Kinetics; D018521:Plant Physiological Phenomena; D010940:Plant Proteins / Q000235:genetics*; D010944:Plants / Q000187:drug effects; D014176:Protein Biosynthesis / Q000187:drug effects; D012093:Replicon
chemical_list: D010940:Plant Proteins; D004247:DNA; D003513:Cycloheximide