Influences of nitrogen deficiency on the transcript levels of ammonium transporter, nitrate transporter and glutamine synthetase genes in Isochrysis galbana (Isochrysidales, Haptophyta)

Abstract

L.-K. Kang, S.-P.L. Hwang, G.-C. Gong, H.-J. Lin, P.-C. Chen and J. Chang. 2007. Influences of nitrogen deficiency on the transcript levels of ammonium transporter, nitrate transporter and glutamine synthetase genes in Isochrysis galbana (Isochrysidales, Haptophyta). Phycologia 46: 521–533. DOI: 10.2216/06-44.1Three DNA fragments belonging to genes involved in nitrogen uptake and assimilation, including a nitrate transporter gene (IgNrt2), an ammonium transporter gene (IgAmt), and a glutamine synthetase gene (IgglnII), were obtained from the marine haptophyte, Isochrysis galbana. Based on alignments of the deduced amino acid sequences, the IgNRT2 fragment shared 47% identity with the Cylindrotheca fusiformis (Bacillariophyceae) nitrate transporter, the IgAMT fragment shared 48% identity with the C. fusiformis ammonium transporter, and the IgGSII fragment shared 61% identity with Skeletonema costatum (Bacillariophyceae) glutamine synthetase. Southern hybridization indicated that both IgNrt2 and IgAmt may have more than one copy in the genome of I. galbana, but only a single copy was detected for IgglnII. The transcript abundances of these three genes under various nitrogen-supply conditions were monitored simultaneously by quantitative real-time polymerase chain reaction (PCR). In the presence of ammonium, both IgNrt2 and IgAmt were severely repressed with mean mRNA concentrations of 0.67 and 3.45 µmol (mol 18S rRNA)−1. IgNrt2 and IgAmt mRNA transcripts increased 41-fold and 8.3-fold, respectively, in the presence of nitrate, and increased 160-fold and 15-fold, respectively, in the nitrogen-depleted condition. The mRNA level of IgglnII was low in cells grown in the presence of ammonium [3.98 µmol (mol 18S rRNA)−1], but mild increases in the range of 2- to 4-fold were observed in cells grown in both nitrate-containing and nitrogen-depleted cultures. Our results demonstrated that the expression patterns of IgNrt2, IgAmt, and IgglnII shared a common trend with their homologous genes in diatoms and green algae under various nitrogen-supply conditions. This group of genes could be used as indicators in natural environments to estimate the degree of nitrogen deficiency and chemical forms of nitrogenous nutrients in use on a species-specific basis.