Identification of TogMNAB, an ABC transporter which mediates the uptake of pectic oligomers in Erwinia chrysanthemi 3937.

The bacterium Erwinia chrysanthemi, which causes soft rot disease on various plants, is able to use pectin as a carbon source for growth. Knowledge of the critical step in pectin catabolism which allows the entry of pectic oligomers into the cells is scarce. We report here the first example of a transport system involved in the uptake of pectic oligomers. The TogMNAB transporter of E. chrysanthemi is a member of the ATP-binding cassette (ABC) superfamily. TogM and TogN are homologous to the inner membrane components, TogA exhibits the signature of ABC ATPases and TogB shows similarity with periplasmic ligand-binding proteins. The TogMNAB transporter is a new member of the carbohydrate uptake transporter-1 family (CUT1, TC no. 3.1.1), which is specialized in the transport of complex sugars. The four genes, togM, togN, togA and togB, are apparently co-transcribed in a large operon which also includes the pectate lyase gene pelW. The transcription of the tog operon is induced in the presence of pectic derivatives and is affected by catabolite repression. It is controlled by the KdgR repressor and the CRP activator. The TogMNAB system is able to provide Escherichia coli with the ability to transport oligogalacturonides. In E. chrysanthemi, the TogMNAB system seems to play a major role in switching on the induction of pectin catabolism. TogB also acts as a specific receptor for chemotaxis towards oligogalacturonides. The decreased capacity of maceration of a togM mutant indicates the importance of transport and/or attraction of oligogalacturonides for E. chrysanthemi pathogenicity.