Eight Unexpected Selenoprotein Families in Organometallic Biochemistry in Clostridium difficile, in ABC Transport, and in Methylmercury Biosynthesis.

The bioinformatics of a nine-gene locus, designated selenocysteine-assisted organometallic (SAO), was investigated after identifying six new selenoprotein families and constructing hidden Markov models (HMMs) that find and annotate members of those families. Four are selenoproteins in most SAO loci, including Clostridium difficile. They include two ABC transporter subunits, namely, permease SaoP, with selenocysteine (U) at the channel-gating position, and substrate-binding subunit SaoB. Cytosolic selenoproteins include SaoL, homologous to MerB organomercurial lyases from mercury resistance loci, and SaoT, related to thioredoxins. SaoL, SaoB, and surface protein SaoC (an occasional selenoprotein) share an unusual CU dipeptide motif, which is something rare in selenoproteins but found in selenoprotein variants of mercury resistance transporter subunit MerT. A nonselenoprotein, SaoE, shares homology with Cu/Zn efflux and arsenical efflux pumps. The organization of the SAO system suggests substrate interaction with surface-exposed selenoproteins, followed by import, metabolism that may cleave a carbon-to-heavy metal bond, and finally metal efflux. A novel type of mercury resistance is possible, but SAO instead may support fermentative metabolism, with selenocysteine-mediated formation of organometallic intermediates, followed by import, degradation, and metal efflux. Phylogenetic profiling shows SOA loci consistently co-occur with Stickland fermentation markers but even more consistently with 8Fe-9S cofactor-type double-cubane proteins. Hypothesizing that the SAO system forms organometallic intermediates, we investigated the known methylmercury formation protein families HgcA and HgcB. Both families contained overlooked selenoproteins. Most HgcAs have a CU motif N terminal to their previously accepted start sites. Seeking additional rare and overlooked selenoproteins may help reveal more cryptic aspects of microbial biochemistry.