A mutation in the α1-tubulin gene of Chlamydomonas reinhardtii confers resistance to anti-microtubule herbicides

A mutation in the α1-tubulin gene of Chlamydomonas reinhardtii was isolated by using the amiprophosmethyl-resistant mutation apm1-18 as a background to select new mutants that showed increased resistance to the drug. The upA12 mutation caused twofold resistance to amiprophos-methyl and oryzalin, and twofold hypersensitivity to the microtubule-stabilizing drug taxol, suggesting that the mutation enhanced microtubule stability. The resistance mutation was semi-dominant and mapped to the same interval on linkage group III as the α-tebulin gene. Two-dimensional gel immunoblots of proteins in the mutant cells revealed two electrophoretically altered α-tubulin isoforms, one of which was acetylated and incorporated into microtubules in the axoneme. The mutant isoforms co-segregated with the drug-resistance phenotypes when mutant upA12 was backcrossed to wild-type cells. Two-dimensional gel analysis of in vitro translation products showed that the non-acetylated variant α-tubulin was a primary gene product. DNA sequence analysis of the α-tubulin genes from mutant and wild-type cells revealed a single missense mutation, which predicted a change in codon 24 from tyrosine in wild type to histidine in mutant up A12. This alteration in the predicted amino acid sequence corroborated the ∼+1 basic charge shift observed for the variant α-tubulins. The mutant allele of the α-tubulin gene was designated tua1-1.