NITRATE AND NITRITE ARE TRANSPORTED BY DIFFERENT SPECIFIC TRANSPORT-SYSTEMS AND BY A BISPECIFIC TRANSPORTER IN CHLAMYDOMONAS-REINHARDTII

Nitrate transport mutants from Chlamydomonas reinhardtii and strains d erived from them upon transformation with plasmids containing the C. r einhardtii nar2/Nrt2;1 or nar2/Nrt2;2 genes have been used to study ni trate and nitrite transport systems. Mutants lacking nitrate assimilat ion clustered genes showed a high affinity nitrite transporter activit y (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 60 those in nontransformed cells. Transformants carrying nar2/Nrt2;1 rec overed high affinity nitrate transporter activity (system 1) together with a considerably enhanced nitrite transport activity. Nitrite trans port mediated by system 1 was very sensitive to inhibition by nitrate at mu M concentrations. Results strongly suggest that three nitrate as similation related high affinity transport systems operate in C. reinh ardtii: 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.