Abstract
Arsenic (As) is widespread in the environment and causes numerous health problems. Rhodopseudomonas palustris has been regarded as a good model organism for studying arsenic detoxification since it was first demonstrated to methylate environmental arsenic by conversion to soluble or gaseous methylated species. However, the detailed arsenic resistance mechanisms remain unknown though there are at least three arsenic-resistance operons (ars1, ars2, and ars3) in R. palustris. In this study, we investigated how arsenic multi-operons contributed to arsenic detoxification in R. palustris. The expression of ars2 or ars3 operons increased with increasing environmental arsenite (As(III)) concentrations (up to 1.0 mM) while transcript of ars1 operon was not detected in the middle log-phase (55 h). ars2 operon was actively expressed even at the low concentration of As(III) (0.01 μM), whereas the ars3 operon was expressed at 1.0 μM of As(III), indicating that there was a differential regulation mechanism for the three arsenic operons. Furthermore, ars2 and ars3 operons were maximally transcribed in the early log-phase where ars2 operon was 5.4-fold higher than that of ars3 operon. A low level of ars1 transcript was only detected at 43 h (early log-phase). Arsenic speciation analysis demonstrated that R. palustris could reduce As(V) to As(III). Collectively, strain CGA009 detoxified arsenic by using arsenic reduction and methylating arsenic mechanism, while the latter might occur with the presence of higher concentrations of arsenic.
Highlights
Arsenic (As) is a highly toxic, carcinogenic, clastogenic and teratogenic metalloid (Slyemi and Bonnefoy, 2012)
Our results revealed that the expression of the different arsenic operons were affected by growth cycle: the maximal expressions of both ars2 and ars3 operons in CGA009 appeared at early log-phase (43 h) and maintained at high level during the growth of middle log-phase (55 h)
This study provided a novel insight into arsenic resistance mechanisms in R. palustris CGA009, a member of anoxygenic phototrophic bacteria
Summary
Arsenic (As) is a highly toxic, carcinogenic, clastogenic and teratogenic metalloid (Slyemi and Bonnefoy, 2012). Arsenic occurs primarily as inorganic forms of pentavalent arsenate As(V) and trivalent arsenite As(III), with the latter being regarded as the most mobile and toxic form (Yang et al, 2012). Arsenicals generated from natural and anthropogenic sources are the widely distributed contaminants of freshwater, groundwater and seawater (Stolz et al, 2010; Slyemi and Bonnefoy, 2012; Rodríguez-Lado et al, 2013). Most of bacteria and Molecular regulation of multiple arsenic operons archaea virtually carry arsenic-resistance (ars) genes that potentially confer resistance to As(V) and/or As(III) (Jackson and Dugas, 2003). The phenomenon (widespread occurrence of ars genes) indicates the ubiquitous distribution of this toxic metalloids in nature
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