Deletion of rifampicin-inactivating mono-ADP-ribosyl transferase gene of globally altered gene expression profile that favoured increase in ROS levels and thereby antibiotic resister generation.

The physiological role of mono-ADP-ribosyl transferase (Arr) of , which inactivates rifampicin, remains unclear. An earlier study reported increased expression of during oxidative stress and DNA damage. This suggested a role for Arr in the oxidative status of the cell and its associated effect on DNA damage. Since reactive oxygen species (ROS) influence oxidative status, we investigated whether Arr affected ROS levels in . Significantly elevated levels of superoxide and hydroxyl radical were found in the mid-log phase (MLP) cultures of the knockout strain (-KO) as compared those in the wild-type strain (WT). Complementation of -KO with expression from genomically integrated under its native promoter restored the levels of ROS equivalent to that in WT. Due to the inherently high ROS levels in the actively growing KO, rifampicin resisters with mutations could be selected at 0 hr of exposure itself against rifampicin, unlike in the WT where the resisters emerged at 12 hr of rifampicin exposure. Microarray analysis of the actively growing cultures of -KO revealed significantly high levels of expression of genes from succinate dehydrogenase I and NADH dehydrogenase I operons, which would have contributed to the increased superoxide levels. In parallel, expression of specific DNA repair genes was significantly decreased, favouring retention of the mutations inflicted by the ROS. Expression of several metabolic pathway genes also was significantly altered. These observations revealed that Arr was required for maintaining a gene expression profile that would provide optimum levels of ROS and DNA repair system in the actively growing .