A mutation in the folA promoter delays adaptation to minimal medium by Escherichia coli K-12

In its natural environment, Escherichia coli is subjected to frequent changes in nutrients and physical parameters such as temperature. Adapting to new conditions requires an orderly progression of events. We show that a folA mutant that overexpressed dihydrofolate reductase because of a C to T promoter mutation, adapted very slowly when transferred from rich medium at 37 degrees C to glucose minimal medium at 42 degrees C. It adapted more rapidly when serine and threonine were added. At 37 degrees C, it was more sensitive than its parent to serine, pyruvate and valine. We propose that the mutant does not increase synthesis of serine and threonine normally upon transfer to minimal medium. This results in a limited availability of serine, threonine and isoleucine during adaptation. Phenotypically, this is manifested as a growth delay at 42 degrees C and an increased sensitivity to isoleucine restriction at 37 degrees C.