525 - Hydrogen-mediated mannose uptake in Azotobacter vinelandii

This study looked at Azotobacter vinelandii's ability to use hydrogen gas to power its uptake of the sugar mannose.

What They Saw
They grew A. vinelandii CA in Burk broth but with mannose instead of glucose or sucrose, and either hydrogen or argon in the atmosphere (along with nitrogen and oxygen). They used ¹⁴C mannose to observe its uptake via the radioactivity of the isotope.

The increase in radioactivity from mannose activity was a lot higher in cells given hydrogen than those without, up to 5-fold.

They tried inhibiting respiration to see if that was related to this effect, and found that usually by inhibiting respiration, they could inhibit the increased mannose uptake, so it seems to be respiration-dependent rather than some sort of regulatory effect.

So this seems to be another of hydrogen's possible roles in the energy metabolism of Azotobacter.

Reference:

Maier, R. J. & Prosser, J. Hydrogen-mediated mannose uptake in Azotobacter vinelandii. J. Bacteriol. 170, 1986–1989 (1988).

308 - Hydrogen-mediated enhancement of hydrogenase expression in Azotobacter vinelandii

This study looked at whether added hydrogen could stimulate hydrogenase activity in Azotobacter vinelandii.

What They Saw
They grew cells with or without ammonium, then added argon or hydrogen to their headspace, and measured whole-cell or purified hydrogenase activity. Oxygen or methylene blue were electron acceptors.

With ammonium, there was a little activity, but adding hydrogen gas increased it about 2.5 to 5 times. As a control, injecting the same amount of argon didn't change anything. In nitrogen-fixing cells, adding hydrogen didn't affect activity.

As with others, activity increased over time in the culture, even corrected by biomass; the hypothesis was that excess carbon inhibits it somehow.

If they added an mRNA or protein synthesis inhibitor (rifampin or chloramphenicol) before adding the hydrogen, activity didn't increase with either case, so it seemed like the regulation was transcriptional.

Also, since the effect was the same with methylene blue (which doesn't require electron transport chain components to act as electron acceptor), it seemed that the regulation was at the hydrogenase directly rather than a related component.

Comparing a couple of Mo nitrogenase-deficient strains (CA11 and CA30) to their parent, they saw that hydrogen didn't affect hydrogenase activity in CA much (in nitrogen-fixing conditions), but it did increase the activity a lot in the mutants. The hydrogenase protein abundance increased too. But in conditions with ammonium, CA and CA11 behaved pretty similar.

Reference:

Prosser, J., Graham, L. & Maier, R. J. Hydrogen-mediated enhancement of hydrogenase expression in Azotobacter vinelandii. J. Bacteriol. 170, 1990–1993 (1988).