Nitrate Reductase from a Mutant Strain of Chlamydomonas reinhardii Incapable of Nitrate Assimilation

Nitrate reductase from mutant 305 o f Chlamydomonas reinhardii has been purified about 90-fold and biochemically characterized. The enzyme can use reduced flavins and viologens as electron donors to reduce nitrate but, unlike the nitrate reductase com plex from its parental wild strain, lacks NAD(P)H-nitrate reductase and NAD(P)H-cytochrom e c reductase activities, does not bind to Blue-Agarose or Blue-Sepharose and exhibits a significantly lower molecular weight (177.000 vs. 241.000), whereas its kinetic characteristics and its sensitivity against several inhibitors and treatments are very similar to those o f the terminal nitrate reductase activity o f the wild strain complex. Spectral studies and antagonistic experiments with tungstate show the presence o f cytochrome 6557 and molybdenum. These facts lead us to propose that nitrate reductase from mutant 305 has a protein deletion which affects the pyridine nucleotide binding region o f the diaphorase protein but without any effect on the terminal nitrate reductase activity.