017 - Mo-independent nitrogenase 3 is advantageous for diazotrophic growth of Azotobacter vinelandii on solid medium containing molybdenum

Everyone now knows that Azotobacter vinelandii has three nitrogenases, and that the Mo-containing one is the best. The iron-only nitrogenase is the least efficient but requires the fewest different metals (only iron, obviously, compared to iron plus another for the other two). But does A. vinelandii ever encounter situations in nature where it can't find molybdenum? It seems likely.

In this study, the scientists tested the difference in nitrogen fixation between cells growing on agar plates and cells growing in liquid broth. This is important because in liquid, nutrients are constantly being mixed and cells can all experience the same concentration of them, pretty much; whereas on agar, nutrients don't move around much (they're trapped in the gel) so the concentration around the bacteria decreases as cells use them up.

They tried growing two strains, wild-type CA and strain CA70 which lacks the genes for the iron-only nitrogenase, to see if one or the other grew more quickly on each type of medium. It turned out that, with the same total concentration of Mo in each, on agar CA outgrew CA70 more and more over time, while in liquid the numbers of cells of each strain remained about the same. And when they tested the strains on agar with different concentrations of Mo, CA outgrew CA70 more and more the lower the concentration of Mo was.

To confirm this, they used a strain (CA73) that had a fusion between anfH (one of the iron nitrogenase genes) and lacZ (makes an enzyme that can break down a compound into color, used to determine amount of a gene produced). They tested this on agar or in liquid with different concentrations of Mo, and found that expression of anfH was much higher on agar than in liquid for mid-range concentrations of Mo (on the low end, A. vinelandii expressed anfH in both conditions, while on the high end, it didn't need to express anfH in either).

Finally, they used a 2-D gel to confirm the presence of iron nitrogenase subunits.

So it seems that, when A. vinelandii is growing on agar, it can sometimes deplete the Mo present in the agar to the point that it needs to switch over to use the iron nitrogenase. In the soil, where A. vinelandii is found naturally, it is probably a common occurrence to encounter areas of low Mo and have to switch.

Citation: Maynard, R. H., Premakumar, R. & Bishop, P. E. Mo-independent nitrogenase 3 is advantageous for diazotrophic growth of Azotobacter vinelandii on solid medium containing molybdenum. J. Bacteriol. 176, 5583–5586 (1994).