Abstract
Paracoccus denitrificans has a branched electron transport chain with three terminal oxidases transferring electrons to molecular oxygen, namely aa3-type and cbb3-type cytochrome c oxidases and ba3-type ubiquinol oxidase. In the present study, we focused on strains expressing only one of these enzymes. The competition experiments showed that possession of cbb3-type oxidase confers significant fitness advantage during oxygen-limited growth and supports the biofilm lifestyle. The aa3-type oxidase was shown to allow rapid aerobic growth at a high oxygen supply. Activity of the denitrification pathway that had been expressed in cells grown anaerobically with nitrate was fully inhibitable by oxygen only in wild-type and cbb3 strains, while in strains aa3 and ba3 dinitrogen production from nitrate and oxygen consumption occurred simultaneously. Together, the results highlight the importance of the cbb3-type oxidase for the denitrification phenotype and suggest a way of obtaining novel bacterial strains capable of aerobic denitrification.
Highlights
Denitrifying organisms are, with possible rare exceptions, facultative anaerobes that constitutively use oxygen as the ultimate electron acceptor of their respiratory chain [1,2]
Under low oxygen tensions and in the presence of nitrate the core electron-transfer system is supplemented by the oxidoreductases of denitrification allowing for conversion of nitrate to dinitrogen coupled to the production of biological energy
Because of a number of points of electrons exit, the respiratory chains of denitrifiers are highly branched and their terminal branches compete with one another for electron flow coming from respiratory dehydrogenases
Summary
Denitrifying organisms are, with possible rare exceptions, facultative anaerobes that constitutively use oxygen as the ultimate electron acceptor of their respiratory chain [1,2]. In our early study we observed that the cyanide inhibition curve of TMPD oxidase was biphasic and the proportion of the more resistant component changed inversely with oxygen concentration in the inlet gas entering the culture chamber [9]. This result gave an indication that at low aeration the mitochondrial type (aa3) oxidase is complemented by another enzyme, later identified as a cbb3-type cytochrome c oxidase [10]. The results provide evidence of a relationship between the inhibition of denitrification by oxygen and the presence of a cbb3-type oxidase
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