| Abstract Detail
Metabolism Brown, Kathryn [1], Twing, Katrina [1], Robertson, Deborah [2]. Steps towards unraveling the regulation of nitrogen assimilation in the marine diatom Thalassiosira pseudonana: diurnal oscillations in mRNA levels encoding five key nitrogen-assimilating enzymes. Marine diatoms are dominant members of phytoplankton communities in coastal upwelling systems, reflecting their physiological potential to rapidly exploit upwelled nutrients, particularly nitrate. We identified genes encoding five key nitrogen assimilating enzymes in the genome of the marine diatom Thalassiosira pseudonana and examined the influence of nitrate and ammonium on the diurnal expression of each gene using quantitative reverse transcriptase PCR (QRT-PCR). As in other photosynthetic eukaryotes, nitrate is reduced to nitrite in the cytosol of diatoms by nitrate reductase (NR, encoded by nia). Following a shift to nitrate or ammonium, a diurnal oscillation in nia abundance was observed, similar to patterns reported for NR abundance and activity. The diurnal oscillation was abolished when cells were transferred to continuous light, indicating that nia is not circadian controlled. In contrast to vascular plants, two nitrite reductase genes were identified in the T. pseudonana genome: a chloroplast-targeted ferredoxin-dependent enzyme (nii), homologous to nii of vascular plants, and a cytosolic NAD(P)H-dependent enzyme (nirB), homologous to fungal and bacterial nitrite reductases. Nii was detected in both nitrate- and ammonium-grown cells and the diurnal pattern of expression was correlated with nia levels in each culture condition. In contrast, transcript levels of nirB, and the chloroplast- and cytosolic-targeted glutamine synthetases (glnII and glnN, respectively) were similar in nitrate- and ammonium-grown cells during the first 24 h following the transition to fresh medium. In the following 48 h, nirB, glnII, and glnN abundances were generally higher in nitrate- than ammonium-grown cells, however, patterns of diurnal expression were similar in all cultures. Based on patterns of gene expression, we propose nitrate is assimilated into organic molecules in both the cytosol and chloroplast of diatoms and that the enzymes encoded by nirB and glnN contribute to the ecologically important dark assimilation of nitrate observed in marine diatoms.
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1 - Clark University, Biology
2 - Clark University, Biology, 950 Main Street, Worcester, MA, 01610, USA
Keywords:
nitrogen and carbon metabolism
diatoms.
Presentation Type: Plant Biology Abstract
Session: P
Location: Exhibit Hall (Northeast, Southwest & Southeast)/Hilton
Date: Sunday, July 8th, 2007
Time: 8:00 AM
Number: P19045
Abstract ID:2662 |
