Transcriptional control of hgcAB by an ArsR-like regulator in Pseudodesulfovibrio mercurii ND132

The hgcAB gene pair encodes mercury (Hg) methylation capability in a diverse group of microorganisms, but its evolution and transcriptional regulation remain unknown. Working from the possibility that the evolutionary function of HgcAB may not be Hg methylation, we test a possible link to arsenic resistance. Using model Hg-methylator Pseudodesulfovibrio mercurii ND132, we specifically evaluated transcriptional control of hgcAB by a putative ArsR encoded upstream and co-transcribed with hgcAB. This regulator shares homology with ArsR repressors of arsenic resistance and S-adenosyl-homocysteine (SAH) responsive regulators of methionine biosynthesis but is distinct from other ArsR/SahR in Pseudodesulfovibrio mercurii ND132. Using qPCR and RNA-seq analyses we confirmed this ArsR regulates hgcAB transcription, and is responsive to arsenic and SAH. Additionally, RNA-seq indicated a possible link between hgcAB activity and arsenic transformations by Pseudodesulfovibrio mercurii ND132, with significant up-regulation of other ArsR-regulated arsenic resistance operons alongside hgcAB. Interestingly, wild-type ND132 was less sensitive to AsV (but not AsIII) than an hgcAB knockout strain, supporting the idea that hgcAB may be linked to arsenic resistance. Arsenic significantly impacted Hg-methylation rates by ND132, however, responses varied with culture conditions. Differences in growth and overall metabolic activity did not account for arsenic impacts on methylation. One goal of this research is to better predict MeHg production in nature. However, we found that hgcAB gene and transcript abundance was not a good predictor of Hg-methylation rates. Our finding that hgcAB activity is linked to arsenic may hold clues to the possible environmental drivers of horizontal transfer of hgcAB. IMPORTANCEThis work reveals a link between microbial mercury methylation and arsenic resistance and may hold clues to the evolution of mercury methylation genes (hgcAB). Microbes with hgcAB produce methylmercury, a strong neurotoxin that readily accumulates in the food web. This study addresses a critical gap in our understanding about the environmental factors that control hgcAB expression. We show that hgcAB expression is controlled by an ArsR-like regulator responsive to both arsenic and S-adenosyl-homocysteine in our model organism, Pseudodesulfovibrio mercurii ND132. Exposure to arsenic also significantly impacted Pseudodesulfovibrio mercurii ND132 mercury methylation rates. However, expression of hgcAB was not always a good predictor of Hg methylation rates, highlighting the roles of Hg bioavailability and other biochemical mechanisms in methylmercury production. This study improves our understanding of the controls on hgcAB expression which is needed to better predict environmental methylmercury production.