Regulation of chorismate mutase in Saccharomyces cerevisiae

The Saccharomyces cerevisiae ARO7 gene was cloned by screening a wild-type gene bank for complementation of an aro7 auxotrophic mutant. In vitro mutagenesis of the isolated plasmid (pJFB1) gave several transformants resistant to levels of the phenylalanine analogue 2-thienylalanine inhibitory to the wild-type transformant. Chorismate mutase assays indicated that two of the mutants (J14-26IV6 and J14-26IV9) were resistant to feedback inhibition by tyrosine displayed by wild-type strains. Analysis of the effect of other aromatic amino acids on chorismate mutase activity showed that tryptophan counteracted this inhibition. Analysis of the effect of tyrosine in the growth medium on enzyme activity indicated that the wild-type ARO7 gene was repressed by tyrosine, a phenomenon not previously reported. Two of the 2-thienylalanine resistant mutants (J14-26IV3 and J14-26IV9) appeared to be resistant to this repression. Transcriptional analysis confirmed that the level of ARO7 transcript decreased with increasing tyrosine concentration. In stain J14-26IV9 the ARO7 transcript level was not affected. J14-26IV9, therefore, appears to be a double mutant, resistant to both feedback inhibition and repression by tyrosine.