Introducing the ArsR regulated arsenic stimulon

Abstract

The United States EPA ranks arsenic as the number one environmental toxin. Since microorganisms are significant drivers of arsenic toxicity and mobility in nature, it is important to understand how microbes detect and react to arsenic. The microbial arsenic resistance operon (ars) is critical for sensing arsenic in the environment and controlling the cellular response to this toxin. The ars operon is minimally comprised of arsRBC, which codes for an ArsR transcriptional repressor, arsenite effluxer, and an arsenate reductase, respectively, with the operon negatively regulated by the transcriptional repressor, ArsR. Our model organism Agrobacterium tumefaciens 5A carries two ars operons, with each containing two arsR genes. We conducted an RNASeq study to examine the regulatory roles of the encoded four ArsR regulatory proteins as a function of +/- arsenite. We report that the regulatory influence of the ArsR proteins extends well beyond the ars operon, with both activation and repression effects. In addition to the expected arsenic resistance response, many cellular functions were impacted, including: phosphate acquisition/metabolism, sugar transport, chemotaxis, copper tolerance, and iron homeostasis. Each of the ArsR proteins uniquely influenced different sets of genes and an arsR regulatory hierarchy was observed, wherein ArsR1 is auto regulatory and negatively regulates arsR4, ArsR4 activates arsR2, and ArsR2 negatively regulates arsR3. ArsR3 is the least active with respect to number of genes regulated. To summarize, this study provides a more complete understanding of how microbial gene expression and biogeochemical cycling may be influenced by arsenic in the environment.