THE USE OF MUTANTS TO STUDY NITRATE ASSIMILATION IN GREEN MICROALGAE

In Chlamydomonas reinhardtii, seven genes were genetically identified for the nitrate assimilation pathway. They are scattered in the algal genome: nit1 encodes the NR apoprotein, nit2 a positive-acting regulat ory product, and five more genes are related to the molybdopterin cofa ctor biosynthesis. nit1 is clustered together with four additional gen es related to nitrate assimilation (nar),, which corregulate with nit1 . Mutants which have some of the nitrate assimilation clustered genes and which lack some of them were obtained and characterized. Nitrate t ransport seems to be mediated in C. reinhardtii by two systems each co ntaining the NAR2 product and either NAR3 or NAR-4 which are highly hy drophobic proteins. A different system appears to account for nitrite transport. Expression of nit1 was dependent on the presence of nitrate and light in cells growing in mineral minimum medium. However, the re quirement for light was removed in cells capable of utilizing acetate.