Abstract
A novel TnMERI1-like transposon designated as TnMARS1 was identified from mercury resistant Bacilli isolated from Minamata Bay sediment. Two adjacent ars operon-like gene clusters, ars1 and ars2, flanked by a pair of 78-bp inverted repeat sequences, which resulted in a 13.8-kbp transposon-like fragment, were found to be sandwiched between two transposable genes of the TnMERI1-like transposon of a mercury resistant bacterium, Bacillus sp. MB24. The presence of a single transcription start site in each cluster determined by 5′-RACE suggested that both are operons. Quantitative real time RT-PCR showed that the transcription of the arsR genes contained in each operon was induced by arsenite, while arsR2 responded to arsenite more sensitively and strikingly than arsR1 did. Further, arsenic resistance complementary experiments showed that the ars2 operon conferred arsenate and arsenite resistance to an arsB-knocked out Bacillus host, while the ars1 operon only raised arsenite resistance slightly. This transposon nested in TnMARS1 was designated as TnARS1. Multi-gene cluster blast against bacteria and Bacilli whole genome sequence databases suggested that TnMARS1 is the first case of a TnMERI1-like transposon combined with an arsenic resistance transposon. The findings of this study suggested that TnMERI1-like transposons could recruit other mobile elements into its genetic structure, and subsequently cause horizontal dissemination of both mercury and arsenic resistances among Bacilli in Minamata Bay.
Highlights
Horizontal gene transfer is a mechanism of giving-and-taking genetic elements between different species and genera, which is a strategy of organisms for survival and plays an important role in microbial evolution
The results showed that TnARS1 contains a functional arsenic resistance operon, and that TnMARS1 is a newly isolated TnMERI1-like mercury resistance transposon nesting an arsenic resistance transposon
Among the 11 broad-spectrum mercury resistant Bacillus strains isolated from preserved samples of mercury-polluted Minamata Bay sediment, the mercury resistance determinant of Bacillus megaterium MB1 was the most studied [5,7,29]
Summary
Horizontal gene transfer is a mechanism of giving-and-taking genetic elements between different species and genera, which is a strategy of organisms for survival and plays an important role in microbial evolution. Bacillus megaterium MB1, an isolate from Minamata Bay, Japan, contains a broad-spectrum mercury resistance determinant encoded in a chromosomal class II transposon, TnMERI1 [4]. Eleven of the 30 Bacillus strains showed broad-spectrum mercury resistance and contained mer operons identical to that of Bacillus megaterium MB1 [4,6]. By using genome mining approaches, we have found broad-spectrum mercury resistance gene clusters in mercury-polluted Minamata Bay sediment strains and Bacillus sp. Minamata Bay-isolated Bacillus containing broad-spectrum mercury resistance were used to investigate the localization of these mercury resistance transposons. The results showed that TnARS1 contains a functional arsenic resistance operon, and that TnMARS1 is a newly isolated TnMERI1-like mercury resistance transposon nesting an arsenic resistance transposon. The present study suggested that the dissemination of TnMERI1-like transposons might contribute to the recruitment of other mobile genetic elements into the Bacilli of Minamata Bay
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