Expression of a nitrogen-fixation gene encoding a nitrogenase subunit in yeast

Biological nitrogen fixation is catalyzed by nitrogenase, an enzyme complex exclusive to prokaryotes. We used the yeast Saccharomyces cerevisiae to study the synthesis, and subsequently the assembly, of nitrogenase components in a eukaryote. Here, the Klebsiella pneumoniae nifH gene, encoding the subunit of the Fe protein (Kp2) component of nitrogenase, was expressed in S. cerevisiae from the yeast ADHI promoter. The nifH gene product, detected in yeast by immunoblot analysis with anti-Kp2 antibodies, exhibited the same electrophoretic mobility in SDS-polyacrylamide gels as that of the Kp2 subunit synthesized in K. pneumoniae. Estimates of Kp2 antigen and assays of β-galactosidase activity specified by nifH'-'lacZ fusions showed that the level of nifH product was similar in anaerobically and aerobically grown yeast, but varied with different transforming plasmids and in various haploid and diploid yeast strains. A cistron located downstream to nifH in a transcript resembling the polycistronic mRNA of the nifHDKY operon in K. pneumoniae is not translated in yeast.