AbmR is a mycobacterial dual-function transcription factor and ribonucleoprotein with distinct DNA and RNA-binding determinants

The bacterium Mycobacterium tuberculosis (Mtb) must adapt to myriad host-associated stressors. A recently identified transcription factor, AbmR (ATP-binding mcr11-regulator), regulates expression of an essential stress-responsive small RNA (Mcr11) and inhibits the growth of Mtb. Previously, AbmR was found to make 39S complexes of unknown function. Here we report that AbmR 39S complexes are comprised of AbmR and co-purifying RNAs and that RNA-binding inhibits AbmRs DNA-binding function. While AbmR binds DNA and regulates gene expression in a sequence specific manner, RNA-binding is not sequence specific. Amino acid R146 is important for DNA-binding but completely dispensable for RNA-binding and 39S complex formation, establishing that the RNA- and DNA-binding functions of AbmR are distinct. RNA bound by AbmR was protected from RNase digestion, supporting an RNA modulatory function for the 39S complex. We also found that abmR is required for optimal survival during treatment with the ATP-depleting antibiotic bedaquiline, which is associated with extended RNA stability. These data establish a paradigm wherein a transcription factor assembles into large complexes to transition between mutually exclusive DNA-binding gene regulatory and RNA-binding RNA modulatory functions. Our findings indicate that AbmR is a dual-function protein that may have novel RNA regulatory roles in stress adapted Mtb.