Propagation of allosteric changes though the catalytic-regulatory interface of Escherichia coli aspartate transcarbamylase

Each of two previously isolated strains of Escherichia coli containing a single nonsense codon within the pyrB gene was suppressed with four different nonsense suppressors. The kinetic analysis using crude extracts of these nonsense-suppressed strains indicated that the mutant aspartate transcarbamylases had altered cooperativity and affinity for aspartate as judged by the substrate concentration at half of the maximal velocity. The response to the allosteric effectors ATP and CTP is also changed for both the mutant enzymes. Analysis indicates that this region of the interface between the catalytic and regulatory chains is critical for the transmission of both homotropic and heterotropic effects between the subunits of the enzyme.