cDNA cloning and expression of a gene for isocitrate lyase in pumpkin cotyledons

A cDNA clone for isocitrate lyase (ICL) was isolated from a cDNA library prepared from the poly(A)(+) RNA of etiolated pumpkin cotyledons. The cDNA encoded a polypeptide with 576 amino acids, whose sequence is more than 79% identical to those of ICL from other higher plants and contains the C-terminal tripeptide, Ser-Arg-Met, which is a putative targeting signal to microbodies. Immunogold analysis revealed that ICL protein is exclusively localized in microbodies in etiolated pumpkin cotyledons. Double labeling experiments with protein A-gold particles of different sizes showed that ICL protein coexists with a leaf-peroxisomal enzyme, glycolate oxidase, in the microbodies of greening and senescing pumpkin cotyledons, indicating that transformation between glyoxysomes and leaf peroxisomes occurs directly during greening and senescence of the cotyledons. Immunoblot analysis showed that the amount of ICL protein increased markedly during germination and decreased rapidly when seedlings were exposed to illumination, which induced the microbody transition from glyoxysomes to leaf peroxisomes. In senescing cotyledons, the level of the ICL protein and ICL mRNA increased again with the reverse transition of microbodies from leaf peroxisomes to glyoxysomes. Changes in the amount of ICL protein did not correspond to the changes in the level of ICL mRNA during greening and senescence of the cotyledons, an indication that post-transcriptional regulation plays an important role in the microbody transition.