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Metabolic Interlock

THE ROLE OF THE SUBORDINATE TYPE OF ENZYME IN THE REGULATION OF A COMPLEX PATHWAY

From the ¹ From the Department of Microbiology, University of Tennessee Medical Units, Memphis, Tennessee 38103, and the Department of Microbiology, Baylor College of Medicine, Houston, Texas 77025

The activity of chorismate mutase plays a strategic role in mediating the flow of chorismate between the synthesis of tyrosine and phenylalanine on the one hand, and of tryptophan and a number of aromatic vitamins on the other hand. A gene specifying chorismate mutase in strain 23 of Bacillus subtilis was introduced into the genetic background of strain 168 by deoxyribonucleate transformation. The newly introduced chorismate mutase has a specific activity which is 10 times greater than that of strain 168. This hybrid strain was used to study the effect of the altered activity of chorismate mutase upon the routing of common pathway precursors into the divergent terminal branches of the aromatic amino acid pathway of biosynthesis.

1. Compared to strain 168, the hybrid strain (carrying chorismate mutase-H) was more resistant to growth inhibition by low concentrations of d-tyrosine and less resistant to growth inhibition by low concentrations of 5-methyltryptophan. The hybrid strain was resistant to a weak inhibition of growth which may occur in derivatives of strain 168 in the presence of the combination of tryptophan and tyrosine. The hybrid strain also possesses an increased capacity for incorporation of exogenous shikimate.

2. Chorismate mutase was product-inhibited by prephenate, a finding which we suggest is physiologically significant—a part of the pattern of sequential feedback inhibition which regulates the aromatic pathway in B. subtilis.

3. The considerable ability of the hybrid strain to adjust to the unbalancing influence of chorismate mutase-H is attributed to the compensatory capabilities of allosteric and repressive controls existing in the pathway. Accordingly, when these regulatory controls in the terminal branches of the pathway were lost by mutation, the full potential of an alteration in level of chorismate mutase to unbalance the flow of common precursors to the terminal branches of the pathway was unmasked. The dependence of the aromatic pathway in B. subtilis upon the regulatory enzymes in its terminal branches for the appropriate function of chorismate mutase leads us to describe this chorismate mutase as an example of a subordinate enzyme.

4. Hybrid strains of B. subtilis are widely distributed, often inadvertently. The properties of hybrid strains explain a number of findings in the literature.

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				G. Xie, N. O. Keyhani, C. A. Bonner, and R. A. Jensen Ancient Origin of the Tryptophan Operon and the Dynamics of Evolutionary Change Microbiol. Mol. Biol. Rev., September 1, 2003; 67(3): 303 - 342. [Abstract] [Full Text] [PDF]