<p>Engineering crop plants with the biological nitrogen fixation pathway is a longstanding goal of modern agriculture. Dinitrogenase reductase (NifH) is a critical component of the biological nitrogen fixation pathway, with multiple roles in metal cofactor assembly and catalysis. This enzyme must be folded correctly as a soluble homodimer and loaded with the [4Fe-4S] metallocluster for function. Previous studies have found that <i>Klebsiella oxytoca</i> (<i>Ko</i>) and <i>Azotobacter vinelandii</i> (<i>Av</i>) NifHs were mostly insoluble when targeted to plant mitochondria. Here we found that a translational fusion of two <i>Ko</i>NifH or <i>Av</i>NifH monomers, forming <i>Ko</i>NifHH or <i>Av</i>NifHH synthetic dimers, produced a soluble protein when targeted to plant mitochondria and co-expressed with the putative peptidyl-prolyl cis–trans isomerase NifM. <i>Ko</i>NifHH isolated after expression in leaf mitochondria at ambient oxygen showed some acetylene reduction activity, which did not require co-expression of the nitrogenase-specific metallocluster machinery NifS and NifU. This activity increased after iron-sulfur cluster reconstitution in vitro with recombinant NifU. In a parallel study, we tested a translational fusion of a variant iron-only dinitrogenase reductase (<i>Av</i>AnfHv6) monomer that was soluble but not active as-isolated from plant mitochondria (<i>Av</i>AnfHHv6) (Gregg et al. <CitationRef CitationID="CR20">2025a</CitationRef>). <i>Av</i>AnfHHv6 was abundant and fully soluble when isolated from plant mitochondria, like its monomer. <i>Av</i>AnfHHv6 was not active as-isolated but could be largely activated by iron-sulfur cluster reconstitution in vitro. This study demonstrates how translational fusions help improve solubility and have the potential to generate an active NifH enzyme within plant mitochondria.</p>