HERBICIDE RESISTANCE AND GROWTH OF D1 ALA(251) MUTANTS IN CHLAMYDOMONAS

We elucidated the effects of substituting seven amino acids for Ala at residue 251 of the Chlamydomonas reinhardtii D1 protein on herbicide resistance and photoautotrophic growth. Ala(251) has been suggested to play a key role in the structural integrity and function of the strom al loop between transmembrane helices IV and V of D1 and has previousl y been shown to affect resistance to ''classical'' PSII specific herbi cides. Sensitive and rapid microtiter assays were employed to compare herbicide resistance and photoautotrophic growth in the various mutant s. Substitution of Ala(251) by Ile, Leu or Val conferred resistance to the PSTI herbicides atrazine, bromacil and metribuzin but not to DCMU and impaired photoautotrophic growth in high and low light. Compared to an otherwise isogenic wildtype strain, the Ile and Val mutants exhi bited nearly identical levels of herbicide resistance and reduced grow th while the Leu mutant had even slower growth and higher levels of he rbicide resistance. In contrast Cvs, Pro, Ser and Gly mutants were phe notypically indistinguishable from wildtype in terms of herbicide sens itivity and photoautotrophic doubling times. Collectively, the seven A la(251) mutations differed markedly from an Ala mutant (dr-1) at the w ell characterized Ser(264) D1 residue in terms of herbicide resistance and photoautotrophic growth.