Altered amino acid metabolism in Irp mutants of Escherichia coli K12 and their derivatives

摘要

An Escherichia coli Irp mutant, lacking the leucine-responsive regulatory protein and the global response it controls, is deregulated in the expression of many genes, but is nevertheless able to grow in glucose-minimal medium at 37 °C. In the presence of isoleucine and valine, the growth rate of the Irp mutant at 37 °C is significantly increased by exogenous L-serine or L-leucine (or both), suggesting that synthesis of these amino acids is limiting. In the absence of isoleucine and valine, however, growth is severely inhibited by both L-serine and L-leucine. A shift to 42 °C or to anaerobiosis makes the Irp mutant auxotrophic for L-serine. Three double mutants carrying Irp and another known mutation, acquire new auxotrophies: Irp relA, lacking the stringent response to amino acid limitation, requires leucine; Irp ssd with numerous metabolic perturbations and antibiotic resistances, requires serine and leucine; and Irp pnt, lacking pyridine nucleotide transhydrogenase, requires glutamate or aspartate (or the corresponding amides). The Irp mutant, although able to achieve balanced growth in some conditions, is clearly on the edge of a metabolic precipice, unable to tolerate many physiological and genetic perturbations which are inocuous to wild-type E. coli.