Molecular Biology of Nitrate Assimilation in Chlamydomonas ReinhardtII

Abstract

As in other photosynthetic eukaryotes, nitrate assimilation in the green alga Chlamydomonas is a highly regulated process which requires transport of the anion into the cell and its reduction to ammonium catalyzed by nitrate reductase (NR) and nitrite reductase (NiR). Several genes have already been identified and related with NR structure, regulation of NR synthesis and molybdopterin cofactor (MoCo) biosynthesis. The regulation of the amount of NR enzyme seems to occur at the levels of transcription, translation and degradation. The structural gene for NR, nit-1, has been cloned and its identity demonstrated by restriction fragment length polymorphism analysis, sequencing data and complementation of a nit A mutation by transformation. It has been shown that the nit-l transcript levels are repressed in ammonium medium. NR enzyme activity and the NR transcript are derepressed in nitrate medium with similar kinetics. In nitrogen-free medium, however, derepression of the NR transcript occurred but NR activity levels were very low. The nit-l gene is involved in the control of NR mRNA levels. By using molecular approaches, several of the previously reported suggestions for NR regulation have been confirmed.