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 bet genes involved in COS and choline catabolism are usually clustered in the genome with one regulatory gene, betI. Here, we report a novel regulatory mechanism of COS and choline catabolism by two BetIs in the model marine Roseobacter group bacterium Ruegeria pomeroyi DSS-3. The insertion of two unrelated genes divided the R. pomeroyi DSS-3 bet cluster into two parts, with each part having its own regulatory betI. BetI1 deregulates the transcription of the betI1-betC operon and betB in the presence of choline. COS and choline induce the transcription of the structural genes while repressing the regulatory gene of the betI2-betTA 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 betI2 boxes are fine-tuned by the binding of the effector choline. Bioinformatic analysis indicated that two betIs 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 Ruegeria pomeroyi 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.