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Originally published as Genetics Published Articles Ahead of Print on March 31, 2005.

Genetics, Vol. 170, 631-644, June 2005, Copyright © 2005
doi:10.1534/genetics.105.041574

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Spontaneous Mutations in the Ammonium Transport Gene AMT4 of Chlamydomonas reinhardtii

Kwang-Seo Kim, Eithne Feild1, Natalie King2, Takuro Yaoi3, Sydney Kustu4 and William Inwood

Department of Plant and Microbial Biology, University of California, Berkeley, California 94720

4 Corresponding author: Department of Plant and Microbial Biology, 111 Koshland Hall, University of California, Berkeley, CA 94720-3102.
E-mail: kustu{at}nature.berkeley.edu

Evidence in several microorganisms indicates that Amt proteins are gas channels for NH3 and CH3NH2, and this has been confirmed structurally. Chlamydomonas reinhardtii has at least four AMT genes, the most reported for a microorganism. Under nitrogen-limiting conditions all AMT genes are transcribed and Chlamydomonas is sensitive to methylammonium toxicity. All 16 spontaneous methylammonium-resistant mutants that we analyzed had defects in accumulation of [14C]methylammonium. Genetic crosses indicated that 12 had lesions in a single locus, whereas two each had lesions in other loci. Lesions in different loci were correlated with different degrees of defect in [14C]methylammonium uptake. One mutant in the largest class had an insert in the AMT4 gene, and the insert cosegregated with methylammonium resistance in genetic crosses. The other 11 strains in this class also had amt4 lesions, which we characterized at the molecular level. Properties of the amt4 mutants were clearly different from those of rh1 RNAi lines. They indicated that the physiological substrates for Amt and Rh proteins, the only two members of their protein superfamily, are NH3 and CO2, respectively.




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