Differential regulation of the Chlamydomonas Nar1 gene family by carbon and nitrogen.

Six genes of the Nar1 multigene family from Chlamydomonas reinhardtii were identified and are located on chromosomes I, VI, VII, IX, and XII/XIII. The first known member Nar1.1 encodes a chloroplast nitrite transporter that regulates nitrate assimilation according to carbon availability, and data supporting the idea that NAR1 proteins may participate in adjusting both nitrite and carbon utilization by Chlamydomonas cells are presented herein. The protein sequences deduced from their corresponding cDNAs show the typical signature of the FNT family, but also have particular differences: (1) NAR1.1, NAR1.2, and NAR1.5 contain putative chloroplast transit peptides; and (2) NAR1.3 and NAR1.6 have long C-termini. The expression patterns for Nar1 transcripts showed differential responses to changes in nitrogen or carbon status, as well as a particular regulation by the nitrate assimilation regulatory gene Nit2. One gene, Nar1.2, was strongly carbon-regulated independently of Nit2; two genes, Nar1.1 and Nar1.6, were regulated by nitrogen and Nit2; and the other genes, Nar1.3, Nar1.4, and Nar1.5 were independent of Nit2 and responded to nitrogen or carbon treatments in a transient and not easily understandable way. We have used Xenopus oocytes as a heterologous system for functional expression of NAR1.2. The electrophysiological response to HCO3- and NO2- provides evidence that NAR1.2 is involved in both HCO3- and NO2- transport.