2021 Volume 33 Issue 1 Pages 3-7
Chloroplasts contain their own genome (chloroplast DNA), which interact with a wide variety of proteins to form chloroplast nucleoids. Chloroplast nucleoids serve as “chromosomes” in chloroplasts, functioning as a center for DNA replication, repair, and gene expression. With fluorescence microscopy, chloroplast nucleoids are generally detected as spherical structures of submicron order. Our live imaging technology using a microfluidic device has revealed that chloroplast nucleoids undergo a dramatic morphological shift from a spherical structure to a network-like structure during the chloroplast division. This process is presumably required for the equal distribution of chloroplast nucleoids to all daughter chloroplasts during chloroplast division. Analysis of a mutant with aberrantly aggregated chloroplast nucleoids, monokaryotic chloroplast (moc) 1, revealed that activation of MOC1 (= Holliday junction resolvase), is essential for the initiation of this process. The activity of MOC1 could be directly visualized by employing high-speed atomic force microscopy (AFM) and DNA origami techniques. Furthermore, detailed monitoring of chloroplast nucoeoids in moc1 mutants suggested that the hot spots of chloroplast nucleoid replication would be at the peripheral region of chloroplast nucleoids. These findings suggest a model for the structure of the chloroplast nucleoids that consists of a core composed of supercoiled DNA and DNA-binding proteins and a periphery where relaxed DNA, which tends to be a template for replication.
