Strains of Synechocystis sp. PCC 6803 with altered PsaC II. EPR and optical spectroscopic properties of F(A) and F(B) in aspartate, serine, and alanine replacements of cysteines 14 and 51

Yean Sung Jung, Hya R. Vassiliev, Jianping Yu, Lee McIntosh, John H. Golbeck

Research output: Contribution to journalArticle

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Abstract

A psaC deletion mutant of the unicellular cyanobacterium Synechocystis sp. PCC 6803 was utilized to incorporate site-specific amino acid substitutions in the cysteine residues that ligate the F(A) and F(B) iron- sulfur clusters in Photosystem I (PS I). Cysteines 14 and 51 of PsaC were changed to aspartic acid (C14D(PsaC), C51D(PsaC), C14D/C51D(PsaC)), serine (C14S(PsaC), C51S(PsaC)), and alanine (C14A(PsaC), C51A(PsaC)), and the properties of F(A) and F(B) were characterized by electron paramagnetic resonance spectroscopy and time-resolved optical spectroscopy. The C14D(PsaC)-PS I and C14S(PsaC)-PS I complexes showed high levels of photoreduction of F(A) with g values of 2.045, 1.944, and 1.852 after illumination at 15 K, but there was no evidence of reduced F(B) in the g = 2 region. The C51D(PsaC)-PSI and C51S(PSaC)-PS I complexes showed low levels of photoreduction of F(B) with g values of 2.067, 1.931, and 1.881 after illumination at 15 K, but there was no evidence of reduced F(A) in the g = 2 region. The presence of F(B) was inferred in C14D(PsaC)-PS I and CI4S(PsaC)- PS I, and the presence of F(A) was inferred in C51D(PsaC)-PS I and C51S(PsaC)-PS I by magnetic interaction in the photoaccumulated spectra and by the equal spin concentration of the irreversible P700+ cation generated by illumination at 77 K. Flash-induced optical absorbance changes at 298 K in the presence of a fast electron donor indicate that two electron acceptors function after F(X) in the four mutant PS I complexes at room temperature. These data suggest that a mixed-ligand [4Fe-4S] cluster is present in the mutant sites of C14X-PS I and C51X-PSI (where X = D or S), but that the proposed spin state of S = 3/2 renders the resonances undetectable in the g = 2 region. The C14A(PsaC)-PS I, C51A(PsaC)-PS I and C14D/C51D(PsaC)-PS I complexes show only the photoreduction of Fx, consistent with the absence of PsaC. These results show that only those PsaC proteins that contain two [4Fe- 4S] clusters are capable of assembling onto PS I cores in vivo.

Original languageEnglish (US)
Pages (from-to)8040-8049
Number of pages10
JournalJournal of Biological Chemistry
Volume272
Issue number12
DOIs
StatePublished - Apr 8 1997

All Science Journal Classification (ASJC) codes

  • Biochemistry
  • Molecular Biology
  • Cell Biology

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