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

Increased levels of solar ultraviolet (290-320 nm) (UV-B) radiation co uld have profound effects on plant proteins because the aromatic amino acids in proteins absorb strongly in this spectral region. We have in vestigated the effects of UV-B radiation on plant proteins and have ob served a novel 66-kD protein. This product was formed in vivo when Bra ssica napus L. plants grown for 21 d in 65 mu mol m(-2) s(-1) photosyn thetically 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 irradiatio n and accumulated during the next 16 h in UV-B. The 66-kD protein cros s-reacted with an antiserum against the ribulose-1,5-bisphosphate carb oxylase/oxygenase (Rubisco) holoenzyme. Analysis of soluble leaf prote ins revealed that the 66-kD product had a number of isoforms correspon ding closely to those of the large subunit of Rubisco (LSU). Partial p roteolytic digests of the LSU and the 66-kD protein resulted in an equ ivalent pattern of protein fragments, leading to the conclusion that t he 66-kD protein was a photomodified form of the LSU. A similar high m olecular mass variant of Rubisco was observed in soluble protein extra cts from leaves of tomato (Lycopersicon esculentum), tobacco (Nicotian a tabacum), and pea (Pisum sativum L.) plants treated in vive with UV- B, suggesting that it might be a common product, at least among C-3 pl ants. It is interesting that the 66-kD product appears to be generated after incorporation of the LSU into holoenzyme complexes. This conclu sion was drawn from two lines of evidence. First, the LSU variant co-p urified with holoenzyme complexes isolated by nondenaturing polyacryla mide gel electrophoresis. Second, a UV-B-specific 66-kD protein did no t accumulate in a tobacco mutant that synthesizes the Rubisco subunits but does not assemble them into normal holoenzyme complexes.