Expression, assembly, and processing of an active plant ferredoxin-NADP+ oxidoreductase and its precursor protein in Escherichia coli

The flavoprotein ferredoxin-NADP+ reductase (FNR) catalyzes the final step of the photosynthetic electron transport chain, i.e. the reduction of NADP+ by ferredoxin. A cloned FNR cDNA from a pea library (Newman, B., and Gray, J. (1988) Plant Mol. Biol. 10, 511-520) was used to construct plasmids which express the apoenzyme in Escherichia coli. Two recombinant vectors were prepared, one containing the sequence corresponding to the mature enzyme and another including, in addition, the sequence of the transit peptide that directs FNR to the chloroplast. These proteins were expressed as fusion products to the NH2-terminal portion of beta-galactosidase. In both cases, a 35-kDa immunoreactive polypeptide was the major product, suggesting that the proteins were processed in vivo. NH2-terminal sequence determination of the purified recombinant proteins indicate cleavage at positions -1/-2 with respect to the normal processing site in chloroplasts. The processed enzymes showed enzymatic activities and spectral properties that were similar or identical to those of native plant FNR. When a La protease-deficient E. coli strain was used as a host, the expressed FNR precursor was found to be poorly processed, associated to bacterial pellets, and showed no detectable FNR activity. The overall results indicate that acquisition of the native enzyme conformation and assembly of the prosthetic group takes place in the bacterial host, generating an enzyme that is, as far as studied, indistinguishable from plant FNR.