BETWEEN FEAST AND FAMINE - ENDOGENOUS INDUCER SYNTHESIS IN THE ADAPTATION OF ESCHERICHIA-COLI TO GROWTH WITH LIMITING CARBOHYDRATES

Escherichia coli adapted to growth with low carbohydrate concentration s bypassed the requirement for exogenous inducer with at least three w ell-studied sugar regulons. Induction of mgl and gal genes became inde pendent of added galactose in bacteria approaching stationary phase or during continuous culture with micromolar glucose in the medium. Bact eria became independent of exogenous induction because endogenous gala ctose and cyclic AMP (cAMP) pools were sufficient for high expression of mgl and gal genes under glucose limitation. Limitation-stimulated i nduction of mgl was dependent on a functional galETK operon for synthe sis of the inducer galactose. Intracellular galactose levels were maxi mal not during starvation (or slow steady-state growth rates approachi ng starvation) but at fast growth rates with micromolar glucose. The e xtent of mgl/gal induction correlated better with inducer availability than with cAMP concentrations under all conditions tested. Endogenous inducer accumulation represents an adaptation to low-nutrient environ ments, leading to derepression of high-affinity transport systems like Mgl essential for bacterial competitiveness at low nutrient concentra tions.