Transcriptional control ofhgcABby an ArsR-like regulator inPseudodesulfovibrio mercuriiND132

Abstract

ABSTRACTThehgcABgene 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-methylatorPseudodesulfovibrio mercuriiND132, we specifically evaluated transcriptional control ofhgcABby a putative ArsR encoded upstream and co-transcribed withhgcAB. 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 inPseudodesulfovibrio mercuriiND132. Using qPCR and RNA-seq analyses we confirmed this ArsR regulateshgcABtranscription, and is responsive to arsenic and SAH. Additionally, RNA-seq indicated a possible link betweenhgcABactivity and arsenic transformations byPseudodesulfovibrio mercuriiND132, with significant up-regulation of other ArsR-regulated arsenic resistance operons alongsidehgcAB. Interestingly, wild-type ND132 was less sensitive to AsV (but not AsIII) than anhgcABknockout strain, supporting the idea thathgcABmay 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 thathgcABgene and transcript abundance was not a good predictor of Hg-methylation rates. Our finding thathgcABactivity is linked to arsenic may hold clues to the possible environmental drivers of horizontal transfer ofhgcAB.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 withhgcABproduce 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 controlhgcABexpression. We show thathgcABexpression is controlled by an ArsR-like regulator responsive to both arsenic and S-adenosyl-homocysteine in our model organism,Pseudodesulfovibrio mercuriiND132. Exposure to arsenic also significantly impactedPseudodesulfovibrio mercuriiND132 mercury methylation rates. However, expression ofhgcABwas 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 onhgcABexpression which is needed to better predict environmental methylmercury production.