A CYTOCHROME CBB(3) (CYTOCHROME-C) TERMINAL OXIDASE IN AZOSPIRILLUM-BRASILENSE SP7 SUPPORTS MICROAEROBIC GROWTH

Spectral analysis indicated the presence of a cytochrome cbb(3) oxidas e under microaerobic conditions in Azospirillum brasilense Sp7 cells. The corresponding genes (cytNOQP) were isolated by using PCR These gen es are organized in an operon, preceded by a putative anaerobox. The p henotype of an A. brasilense cytN mutant was analyzed. Under aerobic c onditions, the specific growth rate during exponential phase (mu(e)) O f the A. brasilense cytN mutant was comparable to the wild-type specif ic growth rate (mu(e) of approximately 0.2 h(-1)). In microaerobic NH4 +-supplemented conditions, the low respiration of the A. brasilense cy tN mutant affected its specific growth rate (mu(e), Of approximately 0 .02 h(-1)) compared to the wild-type specific growth rate (mu(e) of ap proximately 0.2 h(-1)). Under nitrogen-fixing conditions, both the gro wth rates and respiration of the wild type were significantly diminish ed in comparison to those under NH4+-supplemented conditions. Differen ces in growth rates and respiration between the wild type and the A. b rasilense cytN mutant were less pronounced under these nitrogen-fixing conditions (mu(e), Of approximately 0.03 h(-1) for the wild type and 0.02 h(-1) for the A. brasilense cytN mutant). The nitrogen-fixing cap acity of the A. brasilense cytN mutant was still approximately 80% of that determined for the wild-type strain. This leads to the conclusion that the A. brasilense cytochrome cbb(3) oxidase is required under mi croaerobic conditions, when a high respiration rate is needed, but tha t under nitrogen-fixing conditions the respiration rate does not seem to be a growth-limiting factor.