CurT dosage quantitatively affects thylakoid structure and PSII performance in Synechocystis sp. PCC 6803

Abstract

The photosynthetic thylakoid membrane systems of many oxygenic photosynthesizers form elaborate three-dimensional structures. Curvature Thylakoid (CurT/CURT1) proteins are central regulators of thylakoid architecture in cyanobacteria and plants, driving both grana stacking and the formation of thylakoid convergence zones (TCZs) by promoting membrane curvature while also contributing to green-algal chloroplast and cyanobacterial cell division. While the functional role of grana stacks in terrestrial photosynthesis is well established, the physiological significance of cyanobacterial TCZs remains unclear. Here, we investigate the physiological role of TCZs and thylakoid ultrastructure in Synechocystis sp. PCC 6803 by quantitatively assessing how CurT protein abundance shapes thylakoid membrane organization, TCZ frequency, and thylakoid layering, and how these architectural features relate to photosynthetic efficiency and growth in a set of curT expression mutants. By correlating cellular CurT levels with thylakoid structural and physiological parameters of the system, we define a quantitative relationship between CurT abundance, thylakoid architecture, photosystem II (PSII) performance, and growth. Our results reveal a non-linear threshold-like relationship best captured through a logarithmic regression model: minimal CurT suffices to restore TCZ formation, partially recovering PSII activity and growth rate, while CurT overexpression further enhances late-stage biomass accumulation through an apparently PSII-activity independent mechanism. Together, these findings identify CurT as a dose-dependent key determinant of thylakoid structure and underscore the functional importance of thylakoid membrane architecture for efficient photosynthesis and growth.