Heat stress induces the synthesis of a new form of ribulose-1,5-bisphosphate carboxylase/oxygenase activase in cotton leaves

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC4.1.1.39) activ ase mRNA and protein synthesis were measured in the leaves of cotton (Gossy pium hirsutum L.) plants under control (28 degreesC) or heat-stress (41 deg reesC) conditions. A decline in activase transcript abundance occurred rapi dly during the photoperiod and was unaffected by heat stress. In response t o high temperature, de novo protein synthesis rapidly shifted from mainly e xpression of Rubisco large and small subunits to the major heat-shock prote ins, while de novo synthesis of the constitutively expressed 47- and 43-kDa activase polypeptides was not appreciably altered. However, heat stress in duced the synthesis of a 46-kDa polypeptide that immunoprecipitated with an tibodies monospecific to activase. Expression of the 46-kDa polypeptide cea sed within 1 h of the return of heat-stressed plants to control conditions. Activase precursors of 55 and 51 kDa were detected among the in vitro tran slation products of RNA from control and heat-stressed plants. In addition, a 53-kDa polypeptide that also immunoprecipitated with anti-activase IgG w as among the in vitro translation products of RNA from heat-stressed plants . This putative activase precursor did not occur among the in vitro transla tion products of RNA from plants that had recovered from heat stress. The l evels of the constitutive 47- and 43-kDa activase polypeptides were similar in control and heat-stressed plants. based on immunoblotting with antibodi es to activase. However, a 46-kDa cross-reacting polypeptide was also prese nt in heat-stressed plants and constituted about 5% of the total activase a fter 48 h at high temperature. The identity of the heat-induced 46-kDa poly peptide as activase was confirmed by protein sequencing, which showed that its N-terminal sequence was identical to that of the constitutive 47-kDa ac tivase polypeptide. The presence of multiple isoforms for both the 47- and 43-kDa activase polypeptides on immunoblots of two-dimensional gels and the complex banding pattern on Southern blots together suggest the existence o f more than one activase gene and the possibility that the synthesis of the heat-induced activase polypeptide may be regulated transcriptionally. Indu ction of a new form of activase may constitute a mechanism of photosyntheti c acclimation to heat stress in cotton.