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The glnZ mutant of Azospirillum brasilense (strain 7611) showed only partial recovery (20 to 40%) after 80 min of ammonia-induced nitrogenase switch-off, whereas the wild type recovered totally within 10 min. In contrast, the two strains showed identical anoxic-induced switch-on/switch-off, indicating no cross talk between the two reactivation mechanisms.

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Nitrogenase activity in several diazotrophs is switched off by ammonium and reactivated after consumption. The signaling pathway to this system in Azospirillum brasilense is not understood. We show that ammoniumdependent switch-off through ADP-ribosylation of Fe protein was partial in a glnB mutant of A. brasilense but absent in a glnB glnZ double mutant. Triggering of inactivation by anaerobic conditions was not affected in either mutant. The results suggest that glnB is necessary for full ammonium-dependent nitrogenase switch-off in A. brasilense.Nitrogenase activity was first shown to be reversibly inhibited by low levels of ammonium ions or energy depletion in Rhodopseudomonas palustris by Zumft and Castillo (40); this effect, called nitrogenase switch-off/switch-on, was later shown for Rhodospirillum rubrum by Ludden and coworkers to be due to the ADP-ribosylation of an arginine residue of one of the Fe protein (dinitrogenase reductase) subunits by the enzyme dinitrogenase reductase ADP-ribosyl transferase (DRAT). Removal of the ADP-ribosyl moiety and reactivation of the Fe protein is catalyzed by the dinitrogenase reductase-activating glycohydrolase (DRAG) (12,13,19). Control of nitrogenase activity by ADP-ribosylation in response to the increase of ammonium concentration or energy depletion also occurs in other diazotrophs, such as Rhodobacter capsulatus, Azospirillum brasilense, and Azospirillum lipoferum, most of these belonging to the subclass of alpha-Proteobacteria (9,10,12,25,39). The exceptions are Azotobacter chroococcum (of gammaProteobacteria) and Azoarcus sp. strain BH72 (of beta-Proteobacteria) (20,21), although the presence of DRAT and DRAG has not been clearly established for these last two bacteria.The activities of the enzymes DRAG and DRAT in A. brasilense are opposingly regulated depending on the ammonium concentration (35,36). On addition of ammonium ions to a culture of A. brasilense fixing nitrogen, the DRAT protein is temporarily activated and DRAG is inactivated, resulting in a rapid increase in the

ABSTRACT. In contrast to climacteric fruits, in which ethylene is known to be pivotal, the regulation of ripening in non-climacteric fruits is not well understood. The strawberry is a typical example of a non-climacteric fruit, which has been used as a model system of these types of fruit. In this study, the effect of exogenous ethephon on the expression of ethylene biosynthesis and signaling genes, FaERF2 and FaACO1, was analyzed in the Fragaria ananassa cultivar Camino Real by quantitative real-time polymerase chain reaction, and the physicochemical and phytochemical characteristics of fruits were evaluated in field trials and postharvest tests. Transcript accumulation was influenced by exogenous treatment with ethephon, which affected the pattern of gene expression during different stages of growth and fruit development, with the highest expression occurring during postharvest tests. In addition, ethephon significantly influenced the phytochemical profile of sugars, anthocyanins, phenolic compounds, and vitamin C contents both in the field-and postharvesttreated fruits at different stages. These results indicate that ethylene regulates the phenylpropanoid maturation pathway in strawberry fruit.