COVALENT DIMERIZATION OF RIBULOSE-BISPHOSPHATE CARBOXYLASE SUBUNITS BY UV-RADIATION

The effect of UV radiation (UV-A, UV-B and UV-C) on ribulose bisphosph ate carboxylase from a variety of plant species was examined. The expo sition of plant leaves or the pure enzyme to UV radiation produced a U V-dependent accumulation of a 65 kDa polypeptide (P65). Different appr oaches were utilized to elucidate the origin and structure of P65: ele ctrophoretic and fluorographic analyses of S-35-labelled ribulose bisp hosphate carboxylase exposed to UV radiation and immunological experim ents using antibodies specific for P65, for the large and small subuni ts of ribulose bisphosphate carboxylase and for high-molecular-mass ag gregates of the enzyme. These studies revealed that P65 is a dimer, fo rmed by the covalent, non-disulphide linkage of one small subunit with one large subunit of ribulose bisphosphate carboxylase. For short per iods of time (< 1 h), the amount of P65 formed increased with the dura tion of the exposure to the UV radiation and with the energy of the ra diation applied. Prolonged exposure to UV radiation (1-6 h) resulted i n the formation of high-molecular-mass aggregates of ribulose bisphosp hate carboxylase. Formation of P65 was shown to depend on the native s tate of the protein, was stimulated by inhibitors of enzyme activity, and was inhibited by activators of enzyme activity. A UV-independent a ccumulation of P65 was also achieved by the in vitro incubation of pla nt crude extracts. However, the UV-dependent and the UV-independent fo rmation of P65 seemed to occur by distinct molecular mechanisms. The W -dependent accumulation of P65 was immunologically detected in all spe cies examined, including Lemna minor, Arum italicum, Brassica oleracea , Triticum aestivum, Zea mays, Pisum sativum and Phaseolus vulgaris, s uggesting that it may constitute a universal response to UV radiation, common to all photosynthetic tissues.