Novel regulatory mechanism of choline-O-sulfate and choline catabolism by two BetIs in Alphaproteobacteria.

Choline-O-sulfate (COS) and choline are ubiquitous in the environment, and diverse bacteria catabolize them into glycine betaine for osmoprotection or as a carbon and/or nitrogen source. The characterized genes involved in COS and choline catabolism are usually clustered in the genome with one regulatory gene, . Here, we report a novel regulatory mechanism of COS and choline catabolism by two BetIs in the model marine Roseobacter group bacterium DSS-3. The insertion of two unrelated genes divided the DSS-3 cluster into two parts, with each part having its own regulatory . BetI1 deregulates the transcription of the operon and in the presence of choline. COS and choline induce the transcription of the structural genes while repressing the regulatory gene of the divergon. Two palindromes with one shared flanking sequence in the intergenic fragment of this divergon are recognized by BetI2. The affinities of BetI2 to these two are fine-tuned by the binding of the effector choline. Bioinformatic analysis indicated that two exist widely in members of Alphaproteobacteria. This study elucidates a novel regulatory pattern of COS and choline catabolism in abundant bacteria.IMPORTANCECholine and its sulfonium analog choline-O-sulfate (COS) are ubiquitous, and their catabolism by the bacterial choline-to-glycine betaine pathway generates a potent osmoprotectant, glycine betaine, and also provides carbon and nitrogen sources. In contrast to previously characterized modes executed by one regulatory BetI, in this study, we elucidate a novel regulatory mechanism of COS and choline catabolism by two BetIs in the model marine Roseobacter group bacterium DSS-3. The two BetIs control distinct steps of COS and choline catabolism and respond differently to osmotic stress. This study indicates that the two BetIs regulatory mode is a long-overlooked mechanism adopted by abundant bacteria.