Sequential deletions of photosystem II assembly factors Ycf48, Ycf39 and Pam68 result in progressive loss of autotrophy in the cyanobacterium Synechocystis PCC 6803

The biogenesis of the cyanobacterial photosystem II (PSII) complex requires a number of auxiliary assembly factors that improve efficiency of the process but their precise function is not well understood. To assess a possible synergic action of the Ycf48 and Ycf39 factors acting in early steps of the biogenesis via interaction with the nascent D1 subunit of PSII, we constructed and characterised a double mutant of the cyanobacterium Synechocystis PCC 6803 lacking both these proteins. In addition, we also deleted the ycf39 gene in the double mutant lacking Ycf48 and Pam68, the latter being a ribosomal factor promoting insertion of chlorophyll (Chl) into the CP47 subunit of PSII. The resulting double ΔYcf48/ΔYcf39 and triple ΔYcf48/ΔPam68/ΔYcf39 mutants were deficient in PSII and total Chl, and in contrast to the source mutants, they lost the capacity for autotrophy. Interestingly, autotrophic growth was restored in both of the new multiple mutants by enhancing Chl biosynthesis using a specific ferrochelatase inhibitor. Taking together with the weak radioactive labelling of the D1 protein, these findings can be explained by inhibition of the D1 synthesis caused by the lack and/or incorrect binding of Chl molecules. The results emphasise the key importance of the sufficient Chl supply for the PSII biogenesis and also support the existence of a so far enigmatic regulatory mechanism leading to the reduced overall Chl biosynthesis/accumulation when the PSII assembly is impaired.