Coevolution of transcriptional and allosteric regulation at the chorismatemetabolic branch point of Saccharomyces cerevisiae

Control of transcription and enzyme activities are two interwoven regulator y systems essential for the function of a metabolic node. Saccharomyces cer evisiae strains differing in enzyme activities at the chorismate branch poi nt of aromatic amino acid biosynthesis were constructed by recombinant DNA technology. Expression of an allosterically unregulated, constitutively act ivated chorismate mutase encoded by the ARO7T(226I) (ARO7(c)) allele deplet ed the chorismate pool. The resulting tryptophan limitation caused growth d efects, which could be counteracted only by transcriptional induction of TR P2 encoding the competing enzyme anthranilate synthase. ARO7 expression is not transcriptionally regulated by amino acids. Transcriptional activation of the ARO7(c) allele led to stronger growth retardation upon tryptophan li mitation. The same effect was achieved by removing the competing enzyme ant hranilate synthase, which is encoded by the TRP2 gene, from the transcripti onal control. The allelic situation of ARO7(c) being under general control instead of TRP2 resulted in severe growth defects when cells were starved f or tryptophan. In conclusion, the specific regulatory pattern acting on enz ymatic activities at the first metabolic node of aromatic amino acid biosyn thesis is necessary to maintain proper flux distribution. Therefore, the ev olution of the sophisticated allosteric regulation of yeast chorismate muta se requires as prerequisite (i) that the encoding ARO7 gene is not transcri ptionally regulated, whereas (ii) the transcription of the competing feedba ck-regulated anthranilate synthase-encoding gene is controlled by availabil ity of amino acids.