Structural domains comprising this superfamily share the structure of the rifampin ADP-ribosyltransferase domain found in Arr enzymes which catalyze ADP-ribosylation of rifamycins. This prevents the action of rifamycin antibiotics, rendering them ineffective. The opportunistic pathogen Mycobacterium smegmatis possesses a chromosomally encoded rifampin ADP-ribosyltransferase (Arr-ms). An homologue of Arr, Arr-2, is present in several transposons and integrons found in Gram-negative pathogenic bacteria. The structure of Arr-ms reveals similarity to protein ADP-ribosyltransferases, despite a lack of amino acid sequence similarity PMID:18349144.