Reversible ADP-ribosylation is demonstrated to be a regulatory mechanism in prokaryotes by heterologous expression

The primary product of biological nitrogen fixation, ammonia, reversibly regulates nitrogenase activity in a variety of diazotrophs by a process called "NH4(+)-switch-off/on." Strong correlative evidence from work in Azospirillum lipoferum and Rhodospirillum rubrum indicates that this regulation involves both the inactivation of dinitrogenase reductase by dinitrogenase reductase ADP-ribosyltransferase and the reactivation by dinitrogenase reductase activating glycohydrolase. The genes encoding these two enzymes, draT and draG, have been cloned from these two organisms, so that direct genetic evidence can be marshaled to test this model in vivo. The draT/G system has been transferred to and monitored in the enteric nitrogen-fixing bacterium Klebsiella pneumoniae, an organism normally devoid of such a regulatory mechanism. The expressed draT and draG genes allowed K. pneumoniae to respond to NH4Cl with a reversible regulation of nitrogenase activity that was correlated with the reversible ADP-ribosylation of dinitrogenase reductase in vivo. Thus, the expression of draT and draG genes in K. pneumoniae is necessary and sufficient to support NH4(+)-switch-off/on, and ADP-ribosylation serves as a reversible regulatory mechanism for controlling nitrogenase activity in prokaryotes.

Ammonium inhibition of nitrogenase activity in Herbaspirillum seropedicae

The effect of oxygen, ammonium ion, and amino acids on nitrogenase activity in the root-associated N2-fixing bacterium Herbaspirillum seropedicae was investigated in comparison with Azospirillum spp. and Rhodospirillum rubrum. H. seropedicae is microaerophilic, and its optimal dissolved oxygen level is from 0.04 to 0.2 kPa for dinitrogen fixation but higher when it is supplied with fixed nitrogen. No nitrogenase activity was detected when the dissolved O2 level corresponded to 4.0 kPa. Ammonium, a product of the nitrogenase reaction, reversibly inhibited nitrogenase activity when added to derepressed cell cultures. However, the inhibition of nitrogenase activity was only partial even with concentrations of ammonium chloride as high as 20 mM. Amides such as glutamine and asparagine partially inhibited nitrogenase activity, but glutamate did not. Nitrogenase in crude extracts prepared from ammonium-inhibited cells showed activity as high as in extracts from N2-fixing cells. The pattern of the dinitrogenase and the dinitrogenase reductase revealed by the immunoblotting technique did not change upon ammonium chloride treatment of cells in vivo. No homologous sequences were detected with the draT-draG probe from Azospirillum lipoferum. There is no clear evidence that ADP-ribosylation of the dinitrogenase reductase is involved in the ammonium inhibition of H. seropedicae. The uncoupler carbonyl cyanide m-chlorophenylhydrazone decreased the intracellular ATP concentration and inhibited the nitrogenase activity of whole cells. The ATP pool was not significantly disturbed when cultures were treated with ammonium in vivo. Possible mechanisms for inhibition by ammonium of whole-cell nitrogenase activity in H. seropedicae are discussed.

[Standard of transesophageal atrial pacing to diagnose dual atrioventricular node pathway]

In order to assess the diagnostic methods of dual atrioventricular node pathway (DAVNP), we performed transesophageal atrial pacing in 58 patients with palpitation. These patients were classified into two groups, group A comprising 40 patients without broken A-V conduction curve during pacing, compared with group B of 18 patients with broken A-V conduction curve. In our study, both atypical Wenckebach cycle and 3:2 A-V conduction during the increment atrial pacing (IAP) had no significant difference between the two groups (P greater than 0.05). The maximum increments of SR interval in Wenckebach cycle of group A and B during pacing (Wenckebach delta SRmax) were 83.59 +/- 20.92 ms and 125.00 +/- 32.52 ms respectively (P less than 0.001) and at the cut-off point of Wenckebach delta SRmax at 120 ms the specificity and positive predicative value were very high (96.88% and 90.91%), but sensitivity was not so high (71.43%). The minimum increments of RS interval (delta RSmin) in a greater change of SR interval showed a significant difference between the two groups and the specificity and positive predicative value were also high, but the sensitivity was not so high, either (78.57%). We conclude that the diagnostic value of both Wenckebach delta SRmax and delta RSmin, when the SR interval is of greater change during pacing to the DAVNP, is rather significant. As the sensitivity is very low, isolated atypical Wenckebach phenomenon is not as reliable a diagnostic criterion to the DAVNP as previously supposed.

Regulation of nitrogenase activity by ammonium chloride in Azospirillum spp

Ammonium chloride (greater than or equal to 0.05 mM) effectively and reversibly inhibited the nitrogenase activity of Azospirillum brasilense, Azospirillum lipoferum and Azospirillum amazonense. The glutamine synthetase inhibitor L-methionine-DL- sulfoximine abolished this "switch-off" in A. lipoferum and A. brasilense, but not in A. amazonense. Azaserine, an inhibitor of glutamate synthase, inhibited nitrogenase activity itself. This provides further evidence for glutamine as a metabolite of regulatory importance in the NH4+ switch-off phenomenon. In A. brasilense and A. lipoferum, a transition period before the complete inhibition of nitrogenase activity after the addition of 1 mM ammonium chloride was observed. The in vitro nitrogenase activity also was decreased after treatment with ammonium. During sodium dodecyl sulfate-polyacrylamide gel electrophoresis, a second dinitrogenase reductase (Fe protein) subunit appeared, which migrated in coincidence with the modified subunit of the inactive Fe protein of the nitrogenase of Rhodospirillum rubrum. After the addition of ammonium 32P was incorporated into this subunit of the Fe protein of A. brasilense. In A. amazonense, the inhibition of nitrogenase activity by ammonium was only partial, and no transition period could be observed. The in vitro nitrogenase activity of ammonium-treated cells was not decreased, and no evidence for a modified Fe protein subunit was found. Nitrogenase extracts of A. amazonense were active and had an Fe protein that migrated as a close double band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis.