Analysis of raphidophyte assimilatory nitrate reductase reveals unique domain architecture incorporating a 2/2 hemoglobin
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Eukaryotic assimilatory nitrate reductase (NR) is a multi-domain protein that catalyzes the rate-limiting step in nitrate assimilation. This protein is highly conserved and has been extensively characterized in plants and algae. Here, we report hybrid NRs (NR2-2/2HbN) identified in two microalgal species, Heterosigma akashiwo and Chattonella subsalsa, with a 2/2 hemoglobin (2/2Hb) inserted into the hinge 2 region of a prototypical NR. 2/2Hbs are a class of single-domain heme proteins found in bacteria, ciliates, algae and plants. Sequence analysis indicates that the C-terminal FAD/NADH reductase domain of NR2-2/2HbN retains identity with eukaryotic NR, suggesting that the 2/2Hb domain was inserted interior to the existing NR domain architecture. Phylogenetic analysis supports the placement of the 2/2Hb domain of NR2-2/2HbN within group I (N-type) 2/2Hbs with high similarity to mycobacterial 2/2HbNs, known to convert nitric oxide to nitrate. Experimental data confirms that H. akashiwo is capable of metabolizing nitric oxide and shows that HaNR2-2/2HbN expression increases in response to nitric oxide addition. Here, we propose a mechanism for the dual function of NR2-2/2HbN in which nitrate reduction and nitric oxide dioxygenase reactions are cooperative, such that conversion of nitric oxide to nitrate is followed by reduction of nitrate for assimilation as cellular nitrogen.
KeywordsNitrate reductase Truncated hemoglobin 2/2 Hemoglobin Heterosigma akashiwo Chattonella subsalsa Nitric oxide
This work was supported by the National Science Foundation (IOS-0745102) and the Environmental Protection Agency (STAR-ECOHAB-R83-3221). We would like to thank Lauren Salvitti, Mark Warner, and Tom Hanson (University of Delaware), and Don Stewart for technical and editorial support.
- Baldauf SL (2008) An overview of the phylogeny and diversity of eukaryotes. J Syst Evol 46:263–273Google Scholar
- Du S, Zhang Y, Lin X, Wang Y, Tang C (2008) Regulation of nitrate reductase by nitric oxide in Chinese cabbage pakchoi (Brassica chinensis L.) Plant Cell Environ 31:195–204Google Scholar
- Guillard RRL (1975) Culture of phytoplankton for feeding marine invertebrates. In: Smith WL, Chanley MH (eds) Culture of marine invertebrate animals. Plenum Press, New York, pp 26–60Google Scholar
- Hammer Ø, Harper DAT, Ryan PD (2001) PAST: paleontological statistics software package for education and data analysis. Palaeontologia Electronica 4:1–9Google Scholar
- Padmakumar KB, Thomas LC, Salini TC, John E, Menon NR, Sanjeevan VN (2011) Monospecific bloom of noxious raphidophyte Chattonella marina in the coastal water of South West coast of India. Int J Biosci 1:57–59Google Scholar
- Taylor FJR, Haigh R (1993) The ecology of fish-killing blooms of the chloromonad flagellate Heterosigma in the Strait of Georgia and adjacent waters. In: Smayda TJ, Shimizu Y (eds) Toxic phytoplankton blooms in the sea. Elsevier Science Publishers, pp 705–710Google Scholar
- Zhang Z, Liu C, Wu Z, Xing L, Li P (2006) Detection of nitric oxide in culture media and studies of nitric oxide formation by marine microalgae. Med Sci Monit 12:75–86Google Scholar