Replication and amplification of the small mitochondrial DNAs in a cell suspension of Black Mexican Sweet maize.

The mitochondrial genome of Black Mexican Sweet (BMS) maize includes the principal genome and two transcriptionally active minicircular DNAs of 1913 and 1445 bp. A cell suspension of this line was used to study the biology and replication of the minicircular DNAs. Synthesis of the DNAs was measured by (32)P incorporation; all mitochondrial DNAs (mtDNAs) were synthesized rapidly during logarithmic growth phases, whereas no synthesis could be detected in stationary phase. When stationary phase cultures were placed in fresh medium and incorporation was measured over time, the 1.9-kb minicircle renewed (32)P incorporation prior to incorporation into the 1.4-kb minicircle, the principal mitochondrial genome, or the nuclear genome. Interestingly, plastid DNA renewed incorporation at the same time as the 1.9-kb minicircle. The early replication of the 1.9-kb minicircle relative to the other DNAs increased the copy number of this DNA relative to the other mitochondrial DNAs. The copy number of the minicircular DNAs also varied between leaf cells and the cell suspension. This indicates that components of the mitochondrial genome exhibit differential replication. The ability to follow replication of individual mitochondrial components makes this system valuable for studies of DNA replication.