Multi-omics dissection of transcriptional and post-transcriptional responses in cyanobacterial high-light adaptation.

Understanding how photosynthetic organisms acclimate to excess light is essential for the development of robust chassis for synthetic biology approaches aimed at expanding the photosynthetically active absorption spectrum. High-light (HL) tolerant strains were previously generated by laboratory evolution from a laboratory type (LT) strain of Synechocystis sp. PCC 6803, with tolerance attributed to a small number of specific point mutations. Key mutations affected the NDH-1L complex F1-subunit (NdhF1F124L) and translation elongation factor G2 (EF-G2R461C). Reintroduction of these mutations into the LT background was sufficient to confer HL tolerance. However, the mechanisms by which a limited set of point mutations mediates HL tolerance have remained unclear. Here, integrated transcriptomic and proteomic analyses of HL-tolerant strains reveal a coordinated network of responses underlying HL tolerance. The NdhF1F124L mutation increased the accumulation of NDH-1 complex subunits, likely accounting for the previously observed enhancement of cyclic electron flow. In contrast, EF-G2R461C increased the abundance of multiple functional classes of proteins associated with HL tolerance, while post-transcriptionally reducing the level of the phycobilisome linker protein CpcC2, resulting in a decreased antenna size. Integrated analyses further demonstrated that HL tolerance involves transcriptional regulation of protein abundance, including the maintenance of phosphate metabolism. Consistently, overexpression of two genes from the Pho regulon increased HL tolerance. Overall, this study demonstrates how a small number of point mutations in genes with central cellular functions can reprogram the cyanobacterial cell to achieve enhanced tolerance to HL.