Nitrate and Nitrite Are Transported by Different Specific Transport Systems and by a Bispecific Transporter in Chlamydomonas reinhardtii

Abstract

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.