Pathway and regulation of nitrate reduction in acetogenic Sporomusa ovata

Introduction: Nitrate is known as an alternative electron acceptor in the acetogenic bacterium Sporomusa ovata^[1]. Inspection of the genome sequence and biochemical analyses revealed the presence of a cytochrome c-containing periplasmic nitrate reductase. Moreover, transcriptome analyses revealed nitrite reductase and hydroxylamine reductase genes upregulated in nitrate-grown cells^[2].

Goals: To get insights into the pathway of cytochrome c-dependent nitrate reduction of S. ovata.

Materials & Methods: Bioinformatics, growth experiments, metabolite analyses, resting cells experiments, determination of transcript levels, measurement of enzyme activities.

Results: Nitrate was reduced to ammonia as predicted from the transcriptome analyses. In the absence of ammonia, nitrate was also used as nitrogen source by S. ovata. Nitrate and nitrite reductase activities were measured in cell-free extract, cytoplasm and membrane fractions of S. ovata. These activities were induced only in cells grown in the presence of nitrate. Nitrate reductase (1.9 ± 0.4 U/mg) and nitrite reductase activity (6.1 ± 0.8 U/mg) were found almost exclusively in the membrane fraction. So far, only reduced methylviologen was used as artificial electron donor for nitrate and nitrite reductase. Both enzymes contain c-type cytochromes and the reduced cytochromes are speculated to be the physiological electron donors.

Summary: S. ovata performs an energy-conserving nitrate respiration. Nitrate-induced membrane-bound nitrate and nitrite reductase activities were identified in the membrane fraction of S. ovata. The pathway of nitrate ammonification will be described.

[1] Balk, M., Mehboob, F., van Gelder, A. H., Rijpstra, W. I., Damsté, J. S., Stams, A. J. (2010) Appl Microbiol Biotechnol 88: 595-603

[2] Waschinger, L. M., Müller, V. (2024) Abstract VAAM 2024 "A cytochrome c-containing periplasmic nitrate reductase in the acetogen Sporomusa ovata"