The transcriptional factors MurR and catabolite activator protein regulate N-acetylmuramic acid catabolism in Escherichia coli.
The MurNAc etherase MurQ of Escherichia coli is essential for the catabolism of the bacterial cell wall sugar N-acetylmuramic acid (MurNAc) obtained either from the environment or from the endogenous cell wall (i.e., recycling). High-level expression of murQ is required for growth on MurNAc as the sole source of carbon and energy, whereas constitutive low-level expression of murQ is sufficient for the recycling of peptidoglycan fragments continuously released from the cell wall during growth of the bacteria. Here we characterize for the first time the expression of murQ and its regulation by MurR, a member of the poorly characterized RpiR/AlsR family of transcriptional regulators. Deleting murR abolished the extensive lag phase observed for E. coli grown on MurNAc and enhanced murQ transcription some 20-fold. MurR forms a stable multimer (most likely a tetramer) and binds to two adjacent inverted repeats within an operator region. In this way MurR represses transcription from the murQ promoter and also interferes with its own transcription. MurNAc-6-phosphate, the substrate of MurQ, was identified as a specific inducer that weakens binding of MurR to the operator. Moreover, murQ transcription depends on the activation by cyclic AMP (cAMP)-catabolite activator protein (CAP) bound to a class I site upstream of the murQ promoter. murR and murQ are divergently orientated and expressed from nonoverlapping face-to-face (convergent) promoters, yielding transcripts that are complementary at their 5' ends. As a consequence of this unusual promoter arrangement, cAMP-CAP also affects murR transcription, presumably by acting as a roadblock for RNA polymerase.