PYRROLINE-5-CARBOXYLATE REDUCTASES IN SOYBEAN NODULES - THE ENZYMES IN HOST CYTOSOL AND BACTEROIDS ARE ANTIGENICALLY DISTINCT

A previous report showed that cytosolic fractions of both free-living Bradyrhizobium japonicum and soybean bacteroids exhibited high levels of pyrroline-5-carboxylate reductase (P5CR) activity and summarized ki netic evidence for a common reductase which was distinct from that in nodule host cytosol. In the present study, a polyclonal rabbit antibod y directed against P5CR in nodule host cytosol (alpha-[P5CR.NHC]) was prepared. Recognition of the native reductase by alpha-[P5CR.NHC] was shown by immunoinhibition of the catalytic activity of enzyme from nod ule host cytosol; the P5CR activities in the cytosolic fractions from bacteroids and free-living rhizobia were not affected. Immunoblots sho wed that alpha-[P5CR.NHC] recognized the 30-kDa polypeptide of the pur ified plant reductase, as well as in nodule host cytosol. A 43-kDa pol ypeptide was also bound on immunoblots of Bradyrhizobium japonicum cyt osol, but not of bacteroid cytosol. The 43-kDa species seems not to re present the rhizobial P5CR, but a cross-reacting antigen of currently unknown function which is deleted or modified during infection or bact eroid maturation. Antibody directed against P5CR from Escherichia coli did not recognize the reductase from nodule host cytosol, either by i mmunoinhibition or on immunoblots. Copyright (C) 1997 Elsevier Science Ireland Ltd.