Cooperative regulation by ammonium and ammonium derivatives of nitrite uptake in Chlamydomonas reinhardtii

Ammonium transporter genes in Chlamydomonas: the nitrate-specific regulatory gene Nit2 is involved in Amt1;1 expression

Ammonium transport is a key process in nitrogen metabolism. In the green alga Chlamydomonas, we have characterized molecularly the largest family of ammonium transporters (AMT1) so far described consisting of eight members. CrAmt1 genes have an interesting transcript structure with some very small exons. Differential expression patterns were found for each CrAmt1 gene in response to the nitrogen source by using Real Time PCR. These expression patterns were similar under high and low CO2 atmosphere. CrAmt1;1 expression was characterized in detail. It was repressed in both ammonium and nitrate medium, and strongly expressed in nitrogen-free media. Treatment with a Glutamine synthetase inhibitor released partially repression in ammonium and nitrate suggesting that ammonium and its derivatives participate in the observed repressing effects. By studying CrAmt1;1 expression in mutants deficient at different steps of the nitrate assimilation pathway, it has been shown that nitrate has a double negative effect on this gene expression; one related to its reduction to ammonium, and a second one by itself. This second effect of nitrate was dependent on the functionality of the regulatory gene Nit2, specific for nitrate assimilation. Thus, NIT2 would have a dual role on gene expression: the well-known positive one on nitrate assimilation and a novel negative one on Amt1;1 regulation.

Nitrate and nitrate are transported by different specific transport systems and by a bispecific transporter in Chlamydomonas reinhardtii

Nitrate transport mutants from Chlamydomonas reinhardtii and strains derived from them upon transformation with plasmids containing the C. reinhardtii nar2/Nrt2;1 or nar2/Nrt2;2 genes have been used to study nitrate and nitrite transport systems. Mutants lacking nitrate assimilation clustered genes showed a high affinity nitrite transporter activity (system 3), which was subject to ammonium inhibition and appeared to be independent of a functional nar2 gene. Transformants carrying nar2/Nrt2;2 recovered a high affinity nitrate transporter activity (system 2) and showed nitrite transport activities with properties similar to those in nonrecovered high affinity nitrate transporter activity (system 1) together with a considerably enhanced nitrite transport activity. Nitrite transport mediated by system 1 was very sensitive to inhibition by nitrate at microM concentrations. Results strongly suggest that three nitrate assimilation related high affinity transport systems operate in C. reinhardtii: one specific for nitrite, a second one encoded by nar2/Nrt2;2 specific for nitrate, and another one encoded by nar2/Nrt2;1, which is bispecific for these two anions.

Regulation of nitrite uptake and nitrite reductase expression in Chlamydomonas reinhardtii

Expression of nitrite uptake and nitrite reductase activities has been studied in Chlamydomonas reinhardtii under different nutritional conditions. Both activities were expressed at a low level in derepressed cells (with no nitrogen source) and at a high level in induced cells (with nitrate or nitrite). Nitrate was required for both activities to be maximally expressed. Ammonium-grown cells did not show nitrite uptake capability and had a basal nitrite reductase activity. Nitrite uptake but not nitrite reductase levels decreased very significantly in nitrate-induced cells subject to cycloheximide treatment, which suggests that protein(s) involved in the uptake are under a rapid turnover. Nitrite uptake expression was strongly inhibited by the presence of the glutamine synthetase inhibitor L-methionine-D,L-sulfoximine under either derepression or induction conditions, whereas that of nitrite reductase was not affected under the same conditions. Our results indicate that nitrite uptake expression is regulated primarily by ammonium, and that of nitrite reductase by both ammonium and ammonium derivative(s).

Regulation by ammonium of nitrate and nitrite assimilation in Chlamydomonas reinhardtii

The inhibitor of mRNA synthesis, 6-methylpurine, inhibited nitrate reductase derepression in either ammonium-grown or methylammonium-treated wild-type cells of Chlamydomonas reinhardtii, but not in nitrogen-starved cells. In contrast, 6-methylpurine did not inhibit nitrate reductase synthesis in the methylammonium-resistant mutant 2170 (ma-1) either grown on ammonium, treated with methylammonium or nitrogen starved, but did inhibit the continuous synthesis of nitrate reductase, which required the presence of nitrate in the media. In both wild-type and mutant 2170 grown on ammonium and transferred to nitrate media, cycloheximide immediately prevented nitrate reductase derepression when added either at the beginning or at different times of induction treatment. Unlike wild-type cells, mutant 2170 was able to take up either nitrate or nitrite simultaneously with ammonium in whose presence nitrate and nitrite reductases were synthesized. However, synthesis of nitrate reductase was progressively inhibited in the mutant cells when the intracellular ammonium levels were raised as a result of an increase in either the external pH or the extracellular ammonium concentrations. The results rule out the existence of maturase-like proteins in Chlamydomonas and indicate that ammonium has a double effect on the regulation of nitrate reductase synthesis: (a) it prevents nitrate reductase mRNA production; and (b) it controls negatively the expression of this mRNA.