Chlorophyll and Pheophytin Dephytylating Enzymes Required for Efficient Repair of PSII in Synechococcus elongatus PCC 7942

The Chlorophyll Dephytylase1 (CLD1) and pheophytinase (PPH) proteins of Arabidopsis thaliana are homologous proteins characterized respectively as a dephytylase for chlorophylls (Chls) and pheophytin a (Phein a) and a Phein a-specific dephytylase. Three genes encoding CLD1/PPH homologs (dphA1, dphA2 and dphA3) were found in the genome of the cyanobacterium Synechococcus elongatus PCC 7942 and shown to be conserved in most cyanobacteria. His(6)-tagged DphA1, DphA2 and DphA3 proteins were expressed in Escherichia coli, purified to near homogeneity, and shown to exhibit significant levels of dephytylase activity for Chl a and Phein a. Each DphA protein showed similar dephytylase activities for Chl a and Phein a, but the three proteins were distinct in their kinetic properties, with DphA3 showing the highest and lowest V-max and K-m values, respectively, among the three. Transcription of dphA1 and dphA3 was enhanced under high-light conditions, whereas that of dphA2 was not affected by the light conditions. None of the dphA single mutants of S. elongatus showed profound growth defects under low (50 mu mol photons m(-2) s(-1)) or high (400 mu mol photons m(-2) s(-1)) light conditions. The triple dphA mutant did not show obvious growth defects under these conditions, either, but under illumination of 1,000 mu mol photons m(-2) s(-1), the mutant showed more profound growth retardation compared with wild type (WT). The repair of photodamaged photosystem II (PSII) was much slower in the triple mutant than in WT. These results revealed that dephytylation of Chl a or Phein a or of both is required for efficient repair of photodamaged PSII.

Automated summary

The CLD1 and PPH proteins are characterized as dephytylases for chlorophylls and pheophytin a, respectively. Homologs of CLD1 and PPH were found in cyanobacteria, and these proteins showed similar dephytylase activities but different kinetic properties. Transcription of dphA1 and dphA3 was enhanced under high-light conditions, while dphA2 was not affected. Mutants lacking the DphA proteins did not show significant growth defects, but the triple mutant exhibited more profound growth retardation and slower repair of photodamaged PSII under high-light conditions.