SELECTIVE IN VIVO INACTIVATION OF THE TYROSINE-SENSITIVE 3-DEOXY-D-ARABINO-HEPTULOSONATE 7-PHOSPHATE SYNTHASE FROM ESCHERICHIA COLI K12

The first committed step in the synthesis of aromatic compounds in Escherichia coli is catalyzed by three isoenzymes of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase (7-phospho-2-keto-3-deoxy-D-arabino-heptonate D-erythrose-4-phosphate lyase (pyruvate phosphorylating), E.C. 4.2.1.15). Both the tyrosine-sensitive and the phenylalanine-sensitive isoenzymes are stable in exponentially growing cells. However, the tyrosine-sensitive isoenzyme is selectively inactivated at the onset of the stationary phase of growth whereas the phenylalanine-sensitive isoenzyme activity remains constant. The tryptophan-sensitive isoenzyme activity also declines in E. coli cells but was not further characterized. Inactivation of the tyrosine-sensitive isoenzyme does not depend on culture medium pH. Addition of the aromatic amino acids to stationary phase cells does not influence inactivation. Glucose starvation also initiates inactivation and acetate added to glucose starved cells prevents loss of isoenzyme activity. Chloramphenicol addition to culture medium prior to stationary phase of growth also prevents loss of isoenzyme activity. The inactivation is not readily reversible as stationary phase cells revived by addition of glucose show an induction of tyrosine-sensitive isoenzyme which depends upon de novo synthesis. Antibodies against the tyrosine-sensitive isoenzyme were raised in rabbits, and immunochemical methods were developed to analyze the inactivation. Loss of cross-reactive material parallel with loss of enzymatic activity suggests protein degradation is involved in the inactivation process. The inactivation phenomenon allowed the observation of noncoordinate genetic expression between the two genes aroF and tyrA of the aroF-tyrA operon. Included is a discussion of the physiological significance and potential regulatory mechanism inherent in the inactivation with regard to aromatic compound biosynthesis and cellular adaptation to a new environment