AUTHOR=Navarro-Rodríguez Mónica , Buesa José María , Rubio Luis M. 

TITLE=Genetic and Biochemical Analysis of the Azotobacter vinelandii Molybdenum Storage Protein

JOURNAL=Frontiers in Microbiology

VOLUME=10

YEAR=2019

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2019.00579

DOI=10.3389/fmicb.2019.00579

ISSN=1664-302X

ABSTRACT=<p>The N<sub>2</sub> fixing bacterium <italic>Azotobacter vinelandii</italic> carries a molybdenum storage protein, referred to as MoSto, able to bind 25-fold more Mo than needed for maximum activity of its Mo nitrogenase. Here we have investigated a plausible role of MoSto as obligate intermediate in the pathway that provides Mo for the biosynthesis of nitrogenase iron–molybdenum cofactor (FeMo-co). The <italic>in vitro</italic> FeMo-co synthesis and insertion assay demonstrated that purified MoSto functions as Mo donor and that direct interaction with FeMo-co biosynthetic proteins stimulated Mo donation. The phenotype of an <italic>A. vinelandii</italic> strain lacking the MoSto subunit genes (Δ<italic>mosAB</italic>) was analyzed. Consistent with its role as storage protein, the Δ<italic>mosAB</italic> strain showed severe impairment to accumulate intracellular Mo and lower resilience than wild type to Mo starvation as demonstrated by decreased <italic>in vivo</italic> nitrogenase activity and competitive growth index. In addition, it was more sensitive than the wild type to diazotrophic growth inhibition by W. The Δ<italic>mosAB</italic> strain was found to readily derepress <italic>vnfDGK</italic> upon Mo step down, in contrast to the wild type that derepressed Vnf proteins only after prolonged Mo starvation. The Δ<italic>mosAB</italic> mutation was then introduced in a strain lacking V and Fe-only nitrogenase structural genes (Δ<italic>vnf</italic> Δ<italic>anf</italic>) to investigate possible compensations from these alternative systems. When grown in Mo-depleted medium, the Δ<italic>mosAB</italic> and <italic>mosAB</italic><sup>+</sup> strains showed low but similar nitrogenase activities regardless of the presence of Vnf proteins. This study highlights the selective advantage that MoSto confers to <italic>A. vinelandii</italic> in situations of metal limitation as those found in many soil ecosystems. Such a favorable trait should be included in the gene complement of future nitrogen fixing plants.</p>