PLANT PHYSIOLOGY , Vol 109, Issue 1 221-229, Copyright © 1995 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
In Vivo Photomodification of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Holoenzyme by Ultraviolet-B Radiation (Formation of a 66-Kilodalton Variant of the Large Subunit)
M. I. Wilson, S. Ghosh, K. E. Gerhardt, N. Holland, T. S. Babu, M. Edelman, E. B. Dumbroff and B. M. Greenberg
Department of Biology, University of Waterloo, Waterloo, Ontario N2L 3G1 Canada (M.I.W., S.G., K.E.G., T.S.B., E.B.D., B.M.G.)
Increased levels of solar ultraviolet (290-320 nm) (UV-B) radiation could
have profound effects on plant proteins because the aromatic amino acids in
proteins absorb strongly in this spectral region. We have investigated the
effects of UV-B radiation on plant proteins and have observed a novel 66-kD
protein. This product was formed in vivo when Brassica napus L. plants
grown for 21 d in 65 [mu]mol m-2 s-1 photosynthetically active radiation
were subsequently exposed to 65 [mu]mol m-2 s-1 photosynthetically active
radiation plus UV-B radiation (1.5 [mu]mol m-2 s-1). The protein appeared
after 4 h of UV-B irradiation and accumulated during the next 16 h in UV-B.
The 66-kD protein cross-reacted with an antiserum against the
ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) holoenzyme.
Analysis of soluble leaf proteins revealed that the 66-kD product had a
number of isoforms corresponding closely to those of the large subunit of
Rubisco (LSU). Partial proteolytic digests of the LSU and the 66-kD protein
resulted in an equivalent pattern of protein fragments, leading to the
conclusion that the 66-kD protein was a photomodified form of the LSU. A
similar high molecular mass variant of Rubisco was observed in soluble
protein extracts from leaves of tomato (Lycopersicon esculentum), tobacco
(Nicotiana tabacum), and pea (Pisum sativum L.) plants treated in vivo with
UV-B, suggesting that it might be a common product, at least among C3
plants. It is interesting that the 66-kD product appears to be generated
after incorporation of the LSU into holoenzyme complexes. This conclusion
was drawn from two lines of evidence. First, the LSU variant co-purified
with holoenzyme complexes isolated by nondenaturing polyacrylamide gel
electrophoresis. Second, a UV-B-specific 66-kD protein did not accumulate
in a tobacco mutant that synthesizes the Rubisco subunits but does not
assemble them into normal holoenzyme complexes.
