Effect of Carotenoid Overproduction on Oxygen Tolerance of Nitrogen Fixation in Azospirillum brasilense Sp7

Abstract

Carotenoid-overproducing mutants of Azospirillum brasilense Sp7, which contained about 100 times more carotenoids than the wild-type, were obtained after nitrosoguanidine mutagenesis. Growth studies with one of these mutants in oxygen-controlled batch and continuous cultures revealed a slightly improved oxygen tolerance of nitrogen fixation in the mutant as compared to the wild-type. The production of carotenoids was greatly enhanced by increasing the oxygen concentration under nitrogen-deficient conditions. Although nitrogen fixation was severely inhibited by increased oxygen concentrations, in both the mutant and the wild-type, the mutant showed significantly greater efficiency of nitrogen fixation at 12 PM dissolved oxygen, and it fixed five times more total nitrogen than the wild-type under these conditions. In conclusion, high levels of carotenoids slightly enhanced the oxygen tolerance of Azospirillum brasilense under conditions of oxygen stress, but did not extend the optimum p 0 2 for nitrogen fixation to higher oxygen concentrations.

Oxygen taxis and proton motive force in Azospirillum brasilense

“…There is an obvious advantage of such a mechanism for A. brasilense. This nitrogenfixing, free-living bacterium can fix nitrogen only under conditions in which the oxygen concentration is below 1%, and it is able to maintain a vigorous aerobic metabolism at low oxygen concentrations (17,32). The preferred oxygen concentration for aerotaxis (ϳ3 to 5 M) reported in this study coincides with a preferred oxygen concentration for nitrogen fixation (2 to 8 M) reported for the same strain of A. brasilense (17).…”

“…This nitrogenfixing, free-living bacterium can fix nitrogen only under conditions in which the oxygen concentration is below 1%, and it is able to maintain a vigorous aerobic metabolism at low oxygen concentrations (17,32). The preferred oxygen concentration for aerotaxis (ϳ3 to 5 M) reported in this study coincides with a preferred oxygen concentration for nitrogen fixation (2 to 8 M) reported for the same strain of A. brasilense (17). Thus, our results provide quantitative support for the hypothesis that aerotaxis is an adaptive response to optimize the efficiency of nitrogen fixation (3,18).…”

An extra-cytoplasmic function sigma factor and anti-sigma factor control carotenoid biosynthesis in Azospirillum brasilense

“…Activation of the synthesis of carotenoids in the two Tn5 mutants suggested to us that a repressor of carotenoid biosynthesis had been inactivated in these strains. Although carotenoid-producing mutants of A. brasilense Sp7 have been isolated and their phenotype analysed, the genetic basis of carotenoid production has not been investigated (Hartmann & Hurek, 1988). In this study we have shown that inactivation of the genetic locus encoding anti-s factor in A. brasilense Sp7 leads to carotenoid production in the mutants.…”

“…Besides the enzymes that detoxify superoxide and hydrogen peroxide, carotenoids in A. brasilense strain Cd are implicated in protection against oxidative damage owing to their ability to quench singlet oxygen and possibly oxygen radicals (Hartmann & Hurek, 1988;Nur et al, 1981Nur et al, , 1982. The fact that nitrogen fixation is inhibited in A. brasilense Cd at high dissolved oxygen concentrations upon inhibition of carotenoid synthesis by diphenylamine suggests that carotenoids protect nitrogenase from oxidative damage (Nur et al, 1981).…”

“…The synthesis of carotenoids in these mutants is triggered by oxygen stress (Hartmann & Hurek, 1988), and their presence has been shown to improve oxygen tolerance for nitrogen fixation at high dissolved oxygen concentrations (12 mM) compared with their pigmentless parent.…”