Dual inducer signal recognition by an Mlc homologue

The Mlc transcription factor in Escherichia coli controls the expression of the phosphotransferase system genes implicated in the transport of glucose into the cell. Transport of glucose derepresses Mlc-repressed genes by provoking the sequestration of Mlc to the membrane, via an interaction with the dephosphorylated EIIB domain of the glucose transporter, PtsG. NagC, a paralogue of Mlc in E. coli, regulates the use of the amino sugar N-acetylglucosamine (GlcNAc). Both Mlc and NagC are members of the ROK (Repressors, ORFs and Kinases) family. Vibrio cholerae expresses a close orthologue of Mlc, VC2007, which represses the Mlc target, ptsG, in E. coli. However, VC2007 is not sensitive to growth on glucose but responds to growth on N-acetylglucosamine (GlcNAc). We show that growth on GlcNAc generates two different signals, which relieve VC2007 repression of ptsG in E. coli. The majority of the loss of repression is due to VC2007 interacting with dephosphorylated NagE, the GlcNAc-specific transporter. However, a minor part is due to VC2007 binding GlcNAc6P. These two inducing signals are independent and can be separated by mutations in VC2007 eliminating sensitivity to one or other signal. In addition we show that, although most induction of Mlc-repressed genes is dependent upon the interaction of Mlc with PtsG in E. coli, Mlc can also bind to NagE, but it is not sensitive to GlcNAc6P. These observations shed light on how ROK family homologues have evolved in their ability to sense glucose and GlcNAc and of the shift between recognition of different categories of inducer.