Abstract
Photosystem I (PS I) is a multisubunit membrane protein complex that functions as a light-driven plastocyanin-ferredoxin oxidoreductase. The PsbP domain protein 1 (PPD1; At4g15510) is located in the thylakoid lumen of plant chloroplasts and is essential for photoautotrophy, functioning as a PS I assembly factor. In this work, RNAi was used to suppress PPD1 expression, yielding mutants displaying a range of phenotypes with respect to PS I accumulation and function. These PPD1 RNAi mutants showed a loss of assembled PS I that was correlated with loss of the PPD1 protein. In the most severely affected PPD1 RNAi lines, the accumulated PS I complexes exhibited defects in electron transfer from plastocyanin to the oxidized reaction center P700 (+). The defects in PS I assembly in the PPD1 RNAi mutants also had secondary effects with respect to the association of light-harvesting antenna complexes to PS I. Because of the imbalance in photosystem function in the PPD1 RNAi mutants, light-harvesting complex II associated with and acted as an antenna for the PS I complexes. These results provide new evidence for the role of PPD1 in PS I biogenesis, particularly as a factor essential for proper assembly of the lumenal portion of the complex.
Highlights
The assembly of Photosystem I requires additional protein factors
The original description of the ppd1 insertion mutant was the complete loss of Photosystem I (PS I) accumulation [16], the PsbP domain protein 1 (PPD1) RNAi lines showed a correlated loss of PS I assembly with loss of PPD1
In the presence of small amounts of PPD1, fewer PS I complexes accumulate, and these complexes have defects in coupling of LHC I to PS I as well as in electron transfer from plastocyanin to P700ϩ
Summary
The assembly of Photosystem I requires additional protein factors. Results: Arabidopsis RNAi mutants of PPD1 were characterized for Photosystem I function and assembly. Because of the imbalance in photosystem function in the PPD1 RNAi mutants, light-harvesting complex II associated with and acted as an antenna for the PS I complexes These results provide new evidence for the role of PPD1 in PS I biogenesis, as a factor essential for proper assembly of the lumenal portion of the complex. The accumulation of the PsaF, PsaG, and PsaN subunits and plastocyanin (PC) was the most affected in the PPD1 RNAi plants This observation was consistent with observed defects in LHC I antenna coupling to PS I and electron transfer to P700ϩ. In addition to these PS I defects, LHC II antenna proteins functionally associated with the PS I complexes which did accumulate in the PPD1 RNAi plants. The role of the PPD1 protein as a PS I assembly factor is discussed
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