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.