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# Isolation and characterization of nitrite-reductase-deficient mutants of Chlorella sorokiniana (strain 211-8k)

Nicole Burhenne and Rudolf Tischner
Planta
Vol. 211, No. 3 (August 2000), pp. 440-445
Stable URL: http://www.jstor.org/stable/23386020
Page Count: 6
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## Abstract

A method is presented to isolate mutants of Chlorella sorokiniana with defects in $\mathrm{N}{\mathrm{O}}_{3}^{-}$ metabolism. Three nitrite-reductase (NIR; E.C.1.7.7.1)-deficient mutants were obtained from 500 pinpoint-colony-forming clones. The final screening was performed using $\mathrm{N}{\mathrm{O}}_{3}^{-}$, $\mathrm{N}{\mathrm{O}}_{2}^{-}$ or $\mathrm{N}{\mathrm{H}}_{4}^{+}$ as N-source. The mutants isolated absorb $\mathrm{N}{\mathrm{O}}_{3}^{-}$ with rates close to those measured for the wild type and they excrete $\mathrm{N}{\mathrm{O}}_{2}^{-}$ into the medium. The ratio between $\mathrm{N}{\mathrm{O}}_{3}^{-}$ uptake and $\mathrm{N}{\mathrm{O}}_{2}^{-}$ excretion was 1:1. The sensitivity of $\mathrm{N}{\mathrm{O}}_{3}^{-}$ uptake to $\mathrm{N}{\mathrm{H}}_{4}^{+}$ was reduced in the mutant strains as it was in the N-starved wild type of Chlorella. Nitrate reductase (NR; EC 1.6.6.1) expression and NR activity were slightly reduced compared to the wild type due to feedback regulation in the mutant strains. No NIR protein was found in the three mutants. However, NIR activity was obtained (50% of the wild-type) for one mutant strain. The NIR-deficient mutants and the already available NR-deficient mutants will be promising tools for investigations of the nitrate assimilation pathway on the molecular level and for studies searching for signaling of C and N metabolism by inorganic N-compounds.

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