Abstract
Replication of plastid DNA (ptDNA) is essential for self-duplication of plastid. To investigate cis-acting element and mode for replication, intermediates of ptDNA replication were isolated from the liverwort, Marchantia polymorpha L., suspension-culture cell. No replication bubble intermediate was detected by two-dimensional agarose gel electrophoresis, which indicates the absence of fixed active replication origin in the ptDNA. Replication forks proceeding in the direction opposite to that of rRNA transcription were accumulated at the boundary regions between inverted repeats (IR) and short single copy region. Furthermore, recombination intermediates were also detected in the same region. These intermediates disappeared after displacement of the region, demonstrating that the regions contain cis-acting elements which serve as replication fork barrier (RFB) or sites for homologous recombination. The roll of RFB may be regulation of DNA replication mediated by homologous recombination, or prevention of collision between the transcription apparatus of the rRNA operon and the replication machinery.
