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Protein FrmR acts as a regulator, it can form DNA-protein cross-links with a special promotor PfrmAB to prevent activating downstream gene transcription  
Formaldehyde is able to covalently modify protein FrmR, inhibiting its function, so when formaldehyde exists, promotor PfrmAB is capable of activating downstream gene transcription by the destruction of the promoter PfrmAB- the protein FrmRcross-link.