Preferential replication of repeated DNA sequences in nuclei isolated from soybean cells grown in suspension culture.

Nuclei isolated from suspension cultures of soybean cells are able to replicate DNA in vitro for short periods of time. The reaction is not blocked by dideoxythymidine triphosphate, suggesting that DNA polymerase alpha is involved. Reassociation kinetics, restriction analysis, and isopycnic centrifugation of the DNA synthesized in vitro show that repeated sequences are preferentially replicated in isolated nuclei. However, some repeated sequences are not replicated in vitro, including those coding for ribosomal RNA. The addition of cytoplasmic or yeast extracts clearly stimulates DNA replication in vitro but does not improve the ability of nuclei to replicate single copy DNA. Incubation of cells with cytokinins prior to the isolation of nuclei modifies the pattern of replication of repeated sequences in vitro. These experiments suggest that the replication of single copy and repeated nuclear sequences have different requirements and may be controlled in a different way in plant cells.