ISOLATION OF A CDNA FOR NODULE-ENHANCED PHOSPHOENOLPYRUVATE CARBOXYLASE FROM PEA AND ITS EXPRESSION IN EFFECTIVE AND PLANT-DETERMINED INEFFECTIVE PEA NODULES

A cDNA clone for phosphoenolpyruvate carboxylase (PEPC) was isolated f rom nodules of pea (Pisum sativum L.). The levels of activity, the PEP C polypeptide and the mRNA for PEPC in effective nodules and in ineffe ctive nodules induced on the pea mutant E135 (sym 13) were compared to evaluate the regulation of PEPC activity in pea nodules. The cDNA was 3 222 bp long and contained an open reading frame of 2 901 bp, capabl e of encoding a polypeptide of 967 amino acids. The deduced amino acid sequence was similar to those of other plant PEPCs. The expression of this gene was enhanced in nitrogen-fixing effective pea nodules. The activity of PEPC in the effective nodules increased during development of the plants and the activity was maintained until senescence. This increase was associated with increased levels of the 110 kDa polypepti de and the mRNA for PEPC. By contrast, roots always had lower levels o f activity, the polypeptide and the mRNA for PEPC than effective nodul es. In ineffective E135 nodules, the activity of PEPC increased simila rly to that in effective nodules, but then it decreased to the level i n roots after 2.5 weeks post-planting. However, the levels of the 110 kDa polypeptide and the mRNA for PEPC in ineffective nodules were high er than those in roots, even though ineffective nodules had reduced le vels of the polypeptide and mRNA for PEPC as compared with those in ef fective nodules. These findings indicate that after nodule initiation, the activity of PEPC in pea nodules may be regulated at the transcrip tional level and after translation by a putative factor(s) that govern s the effectiveness of the nodules.