Evolutionary implication of nitrogenase-like proteins in the plant kingdom and prospects for NIF gene tranfer in model eukaryotes

Over the past two decades, strategies for the engineering of nitrogen fixation (nif) genes into higher plants plastids have been proposed, but the O2 sensitivity of the nitrogenase component proteins has been considered to be a major problem because nitrogen fixation is not compatible with photosynthetic O2 evolution (1). We have used homologous recombination events to replace precisely the entire chlL coding region in the C. reinhardtii plastome with the eubacterial genes nifH (Figure 1). Surprisingly, nifH can partially restore the capacity for chlorophyll biosynthesis in the dark. Recent in vitro reconstitution of DPOR with purified BchL, BchN-BchB subunits (2) also indicate the structural and mechanistic similarity between DPOR and nitrogenase. Since K. peumoniae nifH can complementarily replace the function of C. reinhardtii chlL, by analogy ChlL might also function in ATP-coupled electron transfer to the other components of the light-independent protochlorophyllide reductase (DPOR) encoded by ChlN and ChlB, which have sequence similarities with NifD and NifK (3). However,the details of the subunit structures, biochemical properties and requirements for biosynthesis of DPOR are as yet largely unknown. It has been proposed that the “chlorophyll iron proteins” evolved from the nitrogenase Fe protein as a consequence of a gene duplication event (4). Our results may have provided in vivo evidence that these two proteins are similar in function as well as structure.