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

Abstract

Three 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 L 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 mu mol (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 ni trogen-depleted condition. The mRNA level of IgglnII was low in cells grown in the presence of ammonium [3.98 mu mol (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

Transcriptomic response of the toxic prymnesiophyte Prymnesium parvum (N. Carter) to phosphorus and nitrogen starvation

“…In this case, an increase of mRNA level of both transporter genes was observed in N-depleted, to nitrate conditioned cells of I. galbana, but when ammonium and nitrate was provided, the expression of both transporter genes decreased. Moreover, the expression of a glutamine synthase gene in I. galbana was upregulated in nitrogen depleted cells, whereas no differential regulation was shown in the presence of ammonium (Kang et al, 2007).…”

“…Surprisingly, no clear N-starvation response was discernable on the gene expression level, because none of the genes putatively involved in N acquisition, transportation and storage were upregulated under N-starvation. In contrast to P. parvum, the non-toxic and presumably non-mixotroph haptophyte I. galbana, reacts to N-starvation by up-regulation of nitrate and ammonium transporters (Kang et al, 2007(Kang et al, , 2009. In this case, an increase of mRNA level of both transporter genes was observed in N-depleted, to nitrate conditioned cells of I. galbana, but when ammonium and nitrate was provided, the expression of both transporter genes decreased.…”

Microalgae Isolation and Selection for Prospective Biodiesel Production

“…Genes that encode for enzymes catalyzing the reduction of NO 3 -to NH 4 + and the biosynthesis of nitrogen-carrying amino acids were strongly expressed under nitrogen limitation [32,33]. Along with the pentose phosphate pathway, these genes were among the most up-regulated genes in -N cells of N. oleoabundans.…”

Diversity of Phytoplankton Nitrate Transporter Sequences from Isolated Single Cells and Mixed Samples from the East China Sea and mRNA Quantification

“…Recently, a group of nitrate transporter genes (Nrt2) was proposed as a potential marker for evaluating nitrogen deficiencies in eukaryotic phytoplankton (23,26,27). Nrt2 genes encode high-affinity nitrate transporters, which are considered the primary transporter system responsible for nitrate uptake (23,26,27,43).…”

“…In addition, genes homologous to Nrt2 were widely identified in bacteria, fungi, algae, and higher plants (for recent reviews, see references 12, 18, and 42). In most marine eukaryotic algae studied, Nrt2 transcript levels are repressed in the presence of ammonium, moderately expressed under a nitrate-sufficient condition, and highly expressed under a nitrogen-deprived condition (26,43). Also, the maximum and minimum transcript levels can be achieved by artificial removal and addition of nitrogenous nutrients, which is helpful in interpreting the physiological state of the original cell population (27).…”