The diversity and functions of choline sulphatases in microorganisms

Choline sulphates have two putative roles in microorganisms: as a reservoir of C, N and S and as osmoprotectants. Although there is no established connection to date regarding the relative distribution of these two functions in microbial communities, this information is crucial in determining the role of choline sulphate in soils, particularly in cultivated soils where S is limiting. Therefore, in order to establish such a connection, the diversity of choline sulphatase (betC) genes was investigated in this study using numerous fully sequenced microbes available in GenBank. Our genomic analyses revealed unequivocally that the betICBA operon is restricted to Rhizobiaceae family members, which live under symbiotic conditions that prevent elemental depletion. Together with the uniform genetic organisation of the betICBA operon in Rhizobiaceae, BetC appears to be both utilised for osmoprotection or S replenishment. In contrast, betC in a wide variety of free-living microbes (including fungi, archaea and bacteria) was found in a cassette encoding only BetC and a choline sulphate transporter, a configuration that appears to be responsible for fulfilling elemental S requirements. Lastly, the relatively high number of BetC sequences available allowed the identification of a specific signature sequence that discriminates between these two functions and also globally defines some conserved motifs in microbial choline sulphatases. Due to the widespread presence of BetC in microbes and the wide repartition of the betC cassette system, the potential importance of choline sulphatase in global S recycling requires further clarification.