Arsenosugars are arsenic-containing ribosides and natural constituents of marine organisms and not toxic to marine plants or animals. Thus the synthesis of arsenosguars represents a mechanism of arsenic detoxification. Arsenosugars also appear to play a central role in the pathway of arsenic biotransformation in marine systems. First, they are likely intermediates in the biosynthesis of the osmolyte arsenobetaine, which is the dominant arsenical in marine organisms and is nontoxic in humans. Second, arsenosugars are also the likely precursors of arsenosugar phospholipids, which may be components of the algal cell membrane. Incorporation of arsenolipids increases under low phosphate conditions and may represent a phosphate-sparing mechanism under those conditions. Arsenosugars were identified more than thirty years ago, and yet their mechanism of biosynthesis remains unknown. S-Adenosylmethionine methyltransferase (ArsM) from Synechocystis sp. PCC 6803 was found to catalyze the formation of a number of methylated intermediates from As(III). However, no direct evidence had been reported to indicate that ArsM was required for arsenosugar biosynthesis. In this study we report identification of the arsM and arsS genes from the cyanobacterium Synechocystis sp. PCC 6803 and show that they are involved in arsenosugar biosynthesis. In the Synechocystis sp. PCC 6803 ars operon, arsS is adjacent to the arsM gene. The gene product, ArsS, contains a characteristic CX₃CX₂C motif which is typical for the radical SAM superfamily. The function of ArsM and ArsS was identified from a combination of arsM or arsS disruption in Synechocystis sp. PCC 6803 and heterologous expression of arsM and arsS in Escherichia coli. Both genes are necessary, indicating a multi-step pathway of arsenosugar biosynthesis. In addition, we demonstrate that ArsS orthologs from three other freshwater cyanobacteria and one picocyanobacterium are involved in arsenosugar biosynthesis in those microbes. This study represents the identification of the first two steps in the pathway of arsenosugar biosynthesis. Our discovery expands the catalytic repertoire of the diverse radical SAM enzyme superfamily and provides a basis for studying the biogeochemistry of complex organoarsenicals.

Keywords: arsenosugar; Synechocystis sp. PCC 6803; arsenic biotransformation