Summary
Log phase cells of Saccharomyces cerevisiae, fully density labelled by growth in 15N medium, were transferred to medium containing only 14N, and DNA replication followed by isopycnic banding in CsCl. Nuclear DNA replicated semi-conservatively, but mitochondrial DNA showed a dispersive pattern, appearing throughout as a single unimodal gradient band the density of which decreased continuously throughout the period of growth. Approaching its first doubling, the mitochondrial component developed marked density heterogeneity, but this was largely dissipated by the time the second doubling was reached. Analysis of single strand densities showed that this phenomenon was an early consequence of a process in which previously incorporated 15N atoms were continually redistributed between old and new DNA strands. By two doublings the operation of this process resulted in the single strands of mitochondrial DNA being indistinguishable in terms of density. The kinetics of density loss of the mitochondrial component showed that incorporation of 15N into mitochondrial DNA following the transfer was negligible and that essentially all pre-incorporated 15N was conserved, indicating the probable absence of metabolic turnover. It was concluded that a primary semi-conservative strand copying step in the replication of mitochondrial DNA is accompanied or closely followed by extensive partial exchanges of homologous strands between duplexes.
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Communicated by B. A. Bridges
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Williamson, D.H., Fennell, D.J. Apparent dispersive replication of yeast mitochondrial DNA as revealed by density labelling experiments. Molec. Gen. Genet. 131, 193–207 (1974). https://doi.org/10.1007/BF00267959
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DOI: https://doi.org/10.1007/BF00267959