Cytochrome c oxidase genes required for nitrogenase activity and diazotrophic growth in Anabaena sp. PCC 7120

Abstract

SummaryN 2 fixation is an O 2 -sensitive process and some filamentous diazotrophic cyanobacteria that grow performing oxygenic photosynthesis confine their N 2 fixation machinery to heterocysts, specialized cells that maintain a reducing environment adequate for N 2 fixation. Respiration is thought to contribute to the diazotrophic metabolism of heterocysts and the genome of the heterocyst-forming cyanobacterium Anabaena sp. PCC 7120 bears three gene clusters putatively encoding cytochrome c oxidases. Transcript analysis of these cox gene clusters through RNA/DNA hybridization identified two cox operons, cox2 and cox3 , that are induced after nitrogen stepdown in an NtcA-and HetR-dependent manner and appear to be expressed specifically in heterocysts. In contrast, cox1 was expressed only in vegetative cells. Expression of cox2 and cox3 occurred at an intermediate stage (about 9 h) during the process of heterocyst development following nitrogen step-down. Inactivation of genes in the two inducible cox operons, but not separately in either of them, strongly reduced nitrogenase activity and prevented diazotrophic growth in aerobic conditions. These results show that the nitrogen-regulated cytochrome c oxidase-type respiratory terminal oxidases Cox2 and Cox3 are essential for heterocyst function in Anabaena sp. PCC 7120.

Experimental and bioinformatic approaches for analyzing and visualizing cyanobacterial nitrogen and hydrogen metabolism

“…While cox1 and cox2 are homologous to aa-type cytochrome c oxidases (Schmetterer, 1994;Jones and Haselkorn, 2002), cox3 is most similar to alternative respiratory terminal oxidases (Valladares et al 2003). The expression of cox2 and cox3 has been reported to be restricted to heterocysts (Valladares et al 2003). In accordance to this result we see no difference in the expression of cox1, while cox2 and cox3 are more strongly expressed under nitrogen fixing conditions (Table 5).…”

“…Nostoc PCC 7120 possesses three cytochrome c oxidase gene clusters, cox1 (alr0950, alr0951, alr0952), cox2 (alr2514, alr2515, alr2516), and cox3 (alr2729, alr2730, alr2731, alr2732, alr2734), respectively (Valladares et al 2003). While cox1 and cox2 are homologous to aa-type cytochrome c oxidases (Schmetterer, 1994;Jones and Haselkorn, 2002), cox3 is most similar to alternative respiratory terminal oxidases (Valladares et al 2003). The expression of cox2 and cox3 has been reported to be restricted to heterocysts (Valladares et al 2003).…”

Adaptation to Oxygen

“…In Anabaena sp. PCC7120, a decrease of photosystem II activity and an increase of aa 3 cytochrome c oxidase activity in the heterocysts has been shown (6,7). For some nonheterocystous cyanobacteria, N 2 fixation takes place in the dark to avoid production of O 2 by photosynthesis (8).…”

Transcriptional regulators ChlR and CnfR are essential for diazotrophic growth in nonheterocystous cyanobacteria

“…Downstream of patB, three genes encoding subunits of cytochrome c oxidase are found in the same direction as nifHDK. These three proteins show the highest similarity to those of the heterocyst-specific type oxidase subunits, CoxB2, CoxA2, and CoxC2, in Anabaena 7120 (8).…”

“…PCC 7120 (Anabaena 7120). Heterocysts are characterized by the following properties: they have a reduced content of photosystem II, the oxygen-evolving complex (6); they develop special cell walls as a physical barrier for oxygen penetration from the environment (7); and they have high respiratory activity by cytochrome c oxidase to remove oxygen and produce ATP to support nitrogenase activity (8).…”