Analysis of the regulatory mechanisms controlling the synthesis of the hexitol transport systems in Escherichia coli K12.

The synthesis of the transport systems (enzymeII-complexes) coded for in the mtl and in the gut (srl) operon was found to be induced by unphosphorylated D-mannitol and D-glucitol respectively. Induction from the outside however is only possible if these polyols are taken up into the cells. Induction of the D-mannitol system is immediate, resistant against catabolite repression, relatively insensitive towards transient repression and starts from a high uninduced level (5--30%). By contrast, the induction of the D-glucitol system starts at a low basal level (0.5--2.5%), does show a pronounced lag from 25 to 90 min, and is hypersensitive towards catabolite and transient repression. These differences apparently reflect primarely differences in the corresponding operator-promotor genes mtl (P,O) and gut (P,O) as well as differences in the uptake of the first, inducing hexitol molecules. For each operon additional regulatory genes exist, called mtlR and gutR respectively, in which transrecessive, temperature sensitive mutations leading to a constitutive expression of the corresponding operon can be found. The influence of these regulatory mechanisms in diauxie experiments and their importance for the differentiation of the three operons during evolution from apparently one common ancestor operon will be discussed.