Freshwater bacteria can methylate selenium through the thiopurine methyltransferase pathway

Involvement of the bacterial thiopurine methyltransferase (bTPMT) in natural selenium methylation by freshwater was investigated. A freshwater environment that had no known selenium contamination but exhibited reproducible emission of dimethyl selenide (DMSe) or dimethyl diselenide (DMDSe) when it was supplemented with an organic form of selenium [(methyl)selenocysteine] or an inorganic form of selenium (sodium selenite) was used. The distribution of the bTPMT gene (tpm) in the microflora was studied. Freshwater bacteria growing on 10 micro M sodium selenite and 10 micro M sodium selenate were isolated, and 4.5 and 10% of the strains, respectively, were shown by colony blot hybridization to hybridize with a Pseudomonas syringae tpm DNA probe. Ribotyping showed that these strains are closely related. The complete rrs sequence of one of the strains, designated Hsa.28, was obtained and analyzed. Its closest phyletic neighbor was found to be the Pseudomonas anguilliseptica rrs sequence. The Hsa.28 strain grown with sodium selenite or (methyl)selenocysteine produced significant amounts of DMSe and DMDSe. The Hsa.28 tpm gene was isolated by genomic DNA library screening and sequencing. BLASTP comparisons of the deduced Hsa.28 bTPMT sequence with P. syringae, Pseudomonas aeruginosa, Vibrio cholerae, rat, and human thiopurine methyltransferase sequences revealed that the levels of similarity were 52 to 71%. PCR-generated Escherichia coli subclones containing the Hsa.28 tpm open reading frame were constructed. E. coli cells harboring the constructs and grown with sodium selenite or (methyl)selenocysteine produced significant levels of DMSe and DMDSe, confirming that the gene plays a role in selenium methylation. The effect of strain Hsa.28 population levels on freshwater DMSe and DMDSe emission was investigated. An increase in the size of the Hsa.28 population was found to enhance significantly the emission of methyl selenides by freshwater samples supplemented with sodium selenite or (methyl)selenocysteine. These data suggest that bTPMT can play a role in natural freshwater selenium methylation processes.