Characterization of SotA and SotB, two Erwinia chrysanthemi proteins which modify isopropyl-beta-D-thiogalactopyranoside and lactose induction of the Escherichia coli lac promoter.

The expression, in Escherichia coli, of variants of the Erwinia chrysanthemi secretion genes outB and outS under the Ptac promoter is toxic to the cells. During attempts to clone E. chrysanthemi genes able to suppress this toxicity, I identified two genes, sotA and sotB, whose products are able to reduce the isopropyl-beta-D-thiogalactopyranoside (IPTG) induction of the E. coli lac promoter. SotA and SotB belong to two different families of the major facilitator superfamily. SotA is a member of the sugar efflux transporter family, while SotB belongs to the multidrug efflux family. The results presented here suggest that SotA and SotB are sugar efflux pumps. SotA reduces the intracellular concentration of IPTG, lactose, and arabinose. SotB reduces the concentration of IPTG, lactose, and melibiose. Expression of sotA and sotB is not regulated by their substrates, but sotA is activated by the cyclic AMP receptor protein (CRP), while sotB is repressed by CRP. Lactose is weakly toxic for E. chrysanthemi. This toxicity is increased in a sotB mutant which cannot efficiently efflux lactose. This first evidence for a physiological role of sugar efflux proteins suggests that their function could be to reduce the intracellular concentration of toxic sugars or sugar metabolites.