Expression of novel nitrate reductase genes in the harmful alga, Chattonella subsalsa

Eukaryotic nitrate reductase (NR) catalyzes the first step in nitrate assimilation and is regulated transcriptionally in response to external cues and intracellular metabolic status. NRs are also regulated post-translationally in plants by phosphorylation and binding of 14-3-3 proteins at conserved serine residues. 14-3-3 binding motifs have not previously been identified in algal NRs. A novel NR (NR2-2/2HbN) with a 2/2 hemoglobin domain was recently described in the alga Chattonella subsalsa. Here, a second NR (NR3) in C. subsalsa is described with a 14-3-3 binding motif but lacking the Heme-Fe domain found in other NRs. Transcriptional regulation of both NRs was examined in C. subsalsa, revealing differential gene expression over a diel light cycle, but not under constant light. NR2 transcripts increased with a decrease in temperature, while NR3 remained unchanged. NR2 and NR3 transcript levels were not inhibited by growth on ammonium, suggesting constitutive expression of these genes. Results indicate that Chattonella responds to environmental conditions and intracellular metabolic status by differentially regulating NR transcription, with potential for post-translational regulation of NR3. A survey of algal NRs also revealed the presence of 14-3-3 binding motifs in other algal species, indicating the need for future research on regulation of algal NRs.