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Gene Review

psaA  -  PSI P700 apoprotein A1

Chlamydomonas reinhardtii

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High impact information on psaA

  • We suggest from these experiments that the product of the tscA gene may be a small chloroplast RNA that acts in trans in the first trans-splicing reaction of psaA [1].
  • In C. reinhardtii, the mature psaA mRNA is assembled by a process involving trans-splicing of three separate transcripts encoded at three widely scattered loci of the chloroplast genome [1].
  • However, in double mutants carrying the nac2-26 mutation, as well as a mutation that prevents psaA splicing, splicing intermediates carrying psbD sequences are degraded [2].
  • Mutant phenotypes support a trans-splicing mechanism for the expression of the tripartite psaA gene in the C. reinhardtii chloroplast [3].
  • The chloroplast psaA gene of the green unicellular alga Chlamydomonas reinhardtii consists of three exons that are transcribed from different strands [3].

Biological context of psaA

  • The abundance of psaA mRNA showed less variation and was maximal around the middle of the cell cycle [4].
  • In Chlamydomonas reinhardtii, the psaA mRNA is assembled by a process involving two steps of trans-splicing that remove two group II introns and give rise to the mature mRNA [5].
  • Maa2 encodes a protein with similarity to conserved domains of pseudouridine synthases, but mutagenesis of putative catalytic residues showed that this activity may not be required for trans-splicing of psaA RNA [5].
  • The downregulation of translation occurs at some early stage since it arises from the interaction between unassembled PsaA and PsaC polypeptides and 5' untranslated regions of psaA and psaC mRNAs, respectively [6].
  • We find no instance of a strain that can perform photosynthesis in the absence of photosystem I. The F8 strain harbored a small amount of photosystem I, and it could fix CO2 and grow slowly, but it lost these abilities after deletion of either psaA or psaC; these activities could be restored to the F8-psaADelta mutant by reintroduction of psaA [7].

Associations of psaA with chemical compounds

  • Although it is not clear whether the pseudouridine synthase activity has been maintained in Maa2, it is possible that this enzyme was recruited during evolution as an RNA chaperone for folding or stabilizing the psaA intron [5].
  • By measuring O2 and CO2 exchange in mutants of the green alga Chlamydomonas reinhardtii in which genes encoding the reaction center of photosystem I (psaA or psaB) have been deleted, we found that a photosystem II-dependent electron flow using O2 as the final acceptor can be sustained in the light [8].

Other interactions of psaA

  • Insertion of these genes into the chloroplast genome through biolistic transformation and analysis of their expression in the TAB1 mutant nuclear background revealed that the psaB (but not the psaA) 5' UTR is the target of the wild-type TAB1 function [9].

Analytical, diagnostic and therapeutic context of psaA

  • Heterologous Cpn60 protein was used in RNA protein gel mobility shift assays and revealed that the ATPase domains of Cpn60 mediates the specific binding of two group II intron RNAs, derived from the homologous chloroplast psaA gene and the heterologous mitochondrial LSU rRNA gene [10].


  1. A small chloroplast RNA may be required for trans-splicing in Chlamydomonas reinhardtii. Goldschmidt-Clermont, M., Choquet, Y., Girard-Bascou, J., Michel, F., Schirmer-Rahire, M., Rochaix, J.D. Cell (1991) [Pubmed]
  2. Mutation at the Chlamydomonas nuclear NAC2 locus specifically affects stability of the chloroplast psbD transcript encoding polypeptide D2 of PS II. Kuchka, M.R., Goldschmidt-Clermont, M., van Dillewijn, J., Rochaix, J.D. Cell (1989) [Pubmed]
  3. Mutant phenotypes support a trans-splicing mechanism for the expression of the tripartite psaA gene in the C. reinhardtii chloroplast. Choquet, Y., Goldschmidt-Clermont, M., Girard-Bascou, J., Kück, U., Bennoun, P., Rochaix, J.D. Cell (1988) [Pubmed]
  4. Cell cycle-dependent transcriptional and post-transcriptional regulation of chloroplast gene expression in Chlamydomonas reinhardtii. Leu, S., White, D., Michaels, A. Biochim. Biophys. Acta (1990) [Pubmed]
  5. A factor related to pseudouridine synthases is required for chloroplast group II intron trans-splicing in Chlamydomonas reinhardtii. Perron, K., Goldschmidt-Clermont, M., Rochaix, J.D. EMBO J. (1999) [Pubmed]
  6. Biogenesis of PSI involves a cascade of translational autoregulation in the chloroplast of Chlamydomonas. Wostrikoff, K., Girard-Bascou, J., Wollman, F.A., Choquet, Y. EMBO J. (2004) [Pubmed]
  7. Photosystem I is indispensable for photoautotrophic growth, CO2 fixation, and H2 photoproduction in Chlamydomonas reinhardtii. Redding, K., Cournac, L., Vassiliev, I.R., Golbeck, J.H., Peltier, G., Rochaix, J.D. J. Biol. Chem. (1999) [Pubmed]
  8. Limited photosynthetic electron flow but no CO2 fixation in Chlamydomonas mutants lacking photosystem I. Cournac, L., Redding, K., Bennoun, P., Peltier, G. FEBS Lett. (1997) [Pubmed]
  9. A nuclear-encoded function essential for translation of the chloroplast psaB mRNA in chlamydomonas. Stampacchia, O., Girard-Bascou, J., Zanasco, J.L., Zerges, W., Bennoun, P., Rochaix, J.D. Plant Cell (1997) [Pubmed]
  10. Chloroplast heat shock protein Cpn60 from Chlamydomonas reinhardtii exhibits a novel function as a group II intron-specific RNA-binding protein. Balczun, C., Bunse, A., Schwarz, C., Piotrowski, M., Kück, U. FEBS Lett. (2006) [Pubmed]
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