Intuitively, it would seem that, if both methylation and demethylation occur in the same organism, a futile cycle could be created. Genomic responses to arsenic in the cyanobacterium Synechocystis sp.
Nevertheless, the genes for arsenic methylation and demethylation coexist in some microorganisms.
Demethylation reverses methylation by converting methylated arsenic into inorganic arsenic.
Since both Ars M and Ars I can use MAs(III) as substrate, they may compete with each other for MAs(III). doi: 10.1371/0096826 Pub Med Abstract | Cross Ref Full Text | Google Scholar Srivastava, A.
Arsenite [As(III)] and methylarsenite [MAs(III)] are the most toxic inorganic and methylated arsenicals, respectively.
As(III) and MAs(III) can be interconverted in the unicellular cyanobacterium Nostoc sp. doi: 10.1007/s11356-013-1741-7 Pub Med Abstract | Cross Ref Full Text | Google Scholar Yan, Y., Ye, J., Xue, X.
Cells co-expressing both genes converted MAs(III) to a mixture of As(III) and DMAs(V).
In Nostoc Nsars M is constitutively expressed, while Nsars I is inducible by either As(III) or MAs(III).
We propose that coexistence of ars M and ars I genes in Nostoc could be advantageous for several reasons.
Because of the unavoidable exposure to arsenic, nearly all of the living organisms have arsenic detoxifying systems (Rosen, 2002; Liu et al., 2013).