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Dinitrogenase is a heterotetrameric (α2β2) enzyme that catalyzes the reduction of dinitrogen to ammonium and contains the iron-molybdenum cofactor (FeMo-co) at its active site. Certain Azotobacter vinelandiimutant strains unable to synthesize FeMo-co accumulate an apo form of dinitrogenase (lacking FeMo-co), with a subunit composition α2β2γ2, which can be activated in vitroby the addition of FeMo-co. The γ protein is able to bind FeMo-co or apodinitrogenase independently, leading to the suggestion that it facilitates FeMo-co insertion into the apoenzyme. In this work, the non-nifgene encoding the γ subunit (nafY) has been cloned, sequenced, and found to encode a NifY-like protein. This finding, together with a wealth of knowledge on the biochemistry of proteins involved in FeMo-co and FeV-co biosyntheses, allows us to define a new family of iron and molybdenum (or vanadium) cluster-binding proteins that includes NifY, NifX, VnfX, and now γ. In vitroFeMo-co insertion experiments presented in this work demonstrate that γ stabilizes apodinitrogenase in the conformation required to be fully activable by the cofactor. Supporting this conclusion, we show that strains containing mutations in both nafYand nifXare severely affected in diazotrophic growth and extractable dinitrogenase activity when cultured under conditions that are likely to occur in natural environments. This finding reveals the physiological importance of the apodinitrogenase-stabilizing role of which both proteins are capable. The relationship between the metal cluster binding capabilities of this new family of proteins and the ability of some of them to stabilize an apoenzyme is still an open matter.

ABSTRACTInfection of severe combined immunodeficient mice withBabesiasp. strain WA1 was studied to assess the contributions of innate and adaptive immunity in resistance to acute babesiosis. The scidmutation showed little effect in genetically susceptible C3H mice and did not decrease the inherent resistance of C57BL/6 mice to the infection, suggesting that innate immunity plays a central role in determining the course ofBabesiainfection in these strains. In contrast, thescidmutation dramatically impaired resistance in moderately susceptible BALB/c mice, suggesting that acquired immunity may play an important secondary role. In comparison to their female counterparts, male mice of different genetic backgrounds showed increased resistance to the infection, indicating that the gender of the host may influence protection against babesiosis.