<p><span>MITH1 Promotes Biased Positioning of Chloroplast Nucleoids Linked to Localized rbcL Expression and Pyrenoid Biogenesis in Chlamydomonas reinhardtii</span></p>

Chloroplast nucleoids are frequently associated with pyrenoids across diverse algal lineages, yet the significance of this conserved spatial relationship has remained unknown. Here, we show that chloroplast nucleoids are significantly and dynamically associated with the pyrenoid in Chlamydomonas reinhardtii. Quantitative imaging demonstrates that pyrenoids preferentially assemble adjacent to pre-existing chloroplast nucleoids during chloroplast division. We further identify MITH1, previously characterized as a pyrenoid tubule biogenesis factor, as a DNA-binding protein that promotes the biased positioning of chloroplast nucleoids toward the base of the chloroplast. Loss of MITH1 disperses chloroplast nucleoids and causes ectopic pyrenoid formation while preserving nucleoid–pyrenoid coupling. MITH1 also promotes the formation of the ring-shaped rbcL mRNA domain surrounding the pyrenoid, linking chloroplast genome organization to localized chloroplast gene expression. Together, our findings establish chloroplast nucleoids as spatial organizers of pyrenoid biogenesis and reveal an unexpected mechanism coordinating chloroplast genome organization with photosynthetic organelle assembly.

Duke Scholars

Author Masayuki Onishi Biology