Evolution of Ycf54-independent chlorophyll biosynthesis in cyanobacteria

Abstract

Significance Photosynthesis uses chlorophylls to utilize solar energy. In oxygenic phototrophs, formation of the isocyclic fifth ring of chlorophyll, responsible for its green color, is catalyzed by AcsF/CycI and the auxiliary protein Ycf54. Removal of Ycf54 causes severe chlorophyll deficiency and impaired photoautotrophic growth. We analyzed laboratory-evolved suppressor mutants of a Ycf54-less strain of the cyanobacterium Synechocystis where chlorophyll biosynthesis and phototrophy were restored. A single point mutation in CycI significantly weakens its dependence on Ycf54, mimicking natural evolution of the enzyme in marine cyanobacteria that lack Ycf54. A second mutation resulting in overaccumulation of chlorophyll inactivates an enzyme with in vitro chlorophyll dephytylase activity. Our results provide insights into the important regulatory role of Ycf54 in chlorophyll biosynthesis.