Modular cyanobacterial regulatory architecture enables dynamic graded responses to oxidative stress

A fundamental paradox of oxygenic photosynthesis: growth-essential energy machinery generates reactive oxygen species (ROS) threatening survival, yet the systems-level regulatory networks balancing the growth-survival trade-off remain unclear. Through integrative experimental-computational analysis combining steady-state transcriptomics with independent component analysis across 0-78.4% oxygen, we decoded the regulatory architecture driving progressive transitions in Synechococcus elongatus PCC 7942 from ROS sensing through defense to growth shutdown. Integration of 407 transcriptome samples identified 78 regulatory modules (iModulons) explaining 72.3% of expression variance and revealed calibrated responses: low stress triggers metalloregulators (SufR, PerR) for ROS sensing and primary antioxidants; moderate stress activates RpaB operating through four distinct regulatory states redirecting metabolism to defense; severe stress induces growth arrest via stringent response pathway convergence. This quantitative regulatory framework enables precise growth-defense calibration through modular network architecture: RpaB coordinates genome-wide resource reallocation (RpaB∼P growth-promoting, RpaB ROS defense-activating, RpaABC circadian-integrating, RpaB ycf46 checkpoint activation), offering systematic strategies for engineering stress-tolerant bioplatforms and predictive models for environmental stress responses.