J. Biol. Chem., Vol. 264, Issue 24, 14129-14135, Aug, 1989
psbG is not a photosystem two gene but may be an ndh gene
PJ Nixon, K Gounaris, SA Coomber, CN Hunter, TA Dyer and J Barber
Agricultural and Food Research Council Photosynthesis Research Group, Imperial College of Science, Technology & Medicine, London, United Kingdom.
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.