ENGINEERED COMPLEMENTATION IN ESCHERICHIA-COLI ASPARTATE TRANSCARBAMOYLASE - HETEROTROPIC REGULATION BY QUATERNARY STRUCTURE STABILIZATION

Escherichia coli aspartate transcarbamoylase regulates pyrimidine bios ynthesis by altering its activity homotropically in response to one of its substrates and heterotropically in response to nucleotide effecte rs, The mechanism of this regulation involves two structurally and fun ctionally different forms of the enzyme, one with low activity and low affinity for substrates (T state) and the other with high activity an d high affinity for substrates (R state), Heterotropic regulation may be due to the direct transmission of a regulatory ''signal'' between t he regulatory site and the active site some 60 Angstrom away and/or ma y involve altering the relative stability of the two forms of the enzy me, By combining a T state-stabilized mutant version of the enzyme, pr eviously thought to have a defect in a heterotropic transmission pathw ay, with a known R state-stabilized mutant enzyme, we were able to res tore some properties of the wild-type enzyme, These data imply that th e relative stabilization of the T and R states of the enzyme is in par t responsible for the homotropic and heterotropic properties of aspart ate transcarbamoylase and that direct pathways for transmission of the heterotropic signals are unlikely.