Abstract
A gene of the chloroplast genome has been designated the psbG gene on the basis that in maize the gene product is a 24-kDa polypeptide of photosystem two (PS2) (Steinmetz, A. A., Castroviejo, M., Sayre, R. T., and Bogorad, L. (1986) J. Biol. Chem. 261, 2485-2488). We have located and sequenced the equivalent gene in wheat (Triticum aestivum) and have raised specific antibodies to the gene product following its expression in Escherichia coli as a beta-galactosidase fusion protein. Using these antibodies, we have investigated the location of the gene product in various thylakoid membrane fractions of pea (Pisum sativum). The gene product of apparent molecular mass 27-28 kDa is severely depleted in PS2-enriched membrane preparations and its distribution between stromal and granal regions of the membrane is distinct to that of the psbC gene product which is known to be a core polypeptide of PS2. We therefore conclude that psbG does not code for a component of PS2 but instead suggest that it is present in a novel protein complex of the thylakoid membrane. On the basis of 1) the conserved overlap between psbG and ndhC, a chloroplast gene which shows significant homology to a mitochondrial gene that codes for a subunit of the NADH-ubiquinone oxidoreductase of mitochondria, and 2) sequence similarity between the psbG gene product and the ndh gene product of E. coli, which codes for a respiratory NADH dehydrogenase, we propose that this ill-defined complex functions as a NADH or NADPH-plastoquinone oxidoreductase.
Highlights
From the $Agriculturaland Food Research Council Photosynthesis Research Group, IlDepartment of Pure h Applied Biology, Imperial College of Science, Technologyh Medicine, London SW7 2BB, United Kingdom and the IlAgriculturaland Food
Location and Nucleotide Sequenceof thepsbG Gene of Wheat (T. aestiuum)- no DNA probe was readily available for the isolation of psbG from wheat, the high conservation of gene order between the chloroplast genomes of maize and wheat [49]suggested that psbG was present in the7.8-kb PstI fragment termed P7 [34]
The primary structure of ORF 245 was found to be greater than 95% similar to that of the psbG gene product of maize and was, concluded to be the wheat psbG gene
Summary
Location and Nucleotide Sequenceof thepsbG Gene of Wheat (T. aestiuum)- no DNA probe was readily available for the isolation of psbG from wheat, the high conservation of gene order between the chloroplast genomes of maize and wheat [49]suggested that psbG was present in the7.8-kb PstI fragment termed P7 [34]. Since ORF 245 is preceded by a sequence similar to the ribosome-binding site of E. coli [50], we consider ORF 245 to represent the true coding region of psbG, there is as yet no evidence to exclude the possibility of initiation 21 bases upstream to yield a protein of 252 residues, or downstream. None of these alternative initiation sitespossess an obvious ribosomebinding site.
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