A cohesive phylogenetic cluster that is limited to enteric bacteria and a few closely related genera possesses a bifunctional protein that is known as the T-protein and is encoded by . The T-protein carries catalytic domains for chorismate mutase and for cyclohexadienyl dehydrogenase. Cyclohexadienyl dehydrogenase can utilize prephenate or L-arogenate as alternative substrates. A portion of the gene cloned from was deleted in with exonuclease III and fused in-frame with a 5' portion of to yield a new gene, denoted *, in which 37 N-terminal amino acids of the T-protein are replaced by 18 amino acids encoded by the polycloning site/5' portion of the α-peptide of pUC19. The TyrA* protein retained dehydrogenase activity but lacked mutase activity, thus demonstrating the separability of the two catalytic domains. While the of the TyrA* dehydrogenase for NADremained unaltered, the for prephenate was fourfold greater and the was almost twofold greater than observed for the parental T-protein dehydrogenase. Activity with L-arogenate, normally a relatively poor substrate, was reduced to a negligible level. The prephenate dehydrogenase activity encoded by * was hypersensitive to feedback inhibition by L-tyrosine (a competitive inhibitor with respect to prephenate), partly because the affinity for prephenate was reduced and partly because the value for L-tyrosine was decreased from 66 μM to 14 μM. Thus, excision of a portion of the chorismate mutase domain is shown to result in multiple extra-domain effects upon the cyclohexadienyl dehydrogenase domain of the bifunctional protein. These include alterations in apparent substrate specificity, isoelectric point, stability, catalytic properties and regulatory properties.


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