Transcriptional Control of the MycobacterialembCABOperon by PknH through a Regulatory Protein, EmbR, In Vivo

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Abstract
EmbR, a putative transcriptional regulator fromMycobacterium tuberculosis, is homologous to the OmpR class of transcriptional regulators that possess winged helix-turn-helix DNA binding motifs. In contrast to other OmpR-like response regulators that are usually phosphorylated and controlled by histidine kinases, EmbR was recently shown to be phosphorylated by the cognate mycobacterial serine/threonine kinase PknH. Despite the in vitro evidence of phosphorylation and interaction between the kinase and regulator, the physiological function of the PknH-EmbR pair is still unknown. We identify theembCABoperon encoding arabinosyltransferases inM. tuberculosisas the cellular target of EmbR. Phosphorylation of EmbR enhances its DNA binding activity towards promoter regions ofembCABgenes. In vivo studies involving expression of PknH inMycobacterium smegmatisestablished its positive regulatory effect on transcription of theembCABoperon via phosphorylation of EmbR. Interestingly, increased transcription ofembC, catalyzing arabinosylation of lipomannan (LM) to lipoarabinomannan (LAM), results in a high LAM/LM ratio, which in turn is a crucial factor in mycobacterial virulence. The PknH-mediated increase in the transcription ofembABgenes significantly alters resistance to ethambutol, a frontline antituberculosis drug known to targetembABgenes. These findings and in vivo upregulation of PknH inside the host macrophages suggest a functionally relevant signaling mechanism involving the PknH-EmbR-embCABsystem.