| Summary: | The widely studied bacterium Rhodobacter sphaeroides 2.4.1 can cleave the abundant anti- stress molecule dimethylsulfoniopropionate into acrylate plus the environmentally important volatile dimethyl sulfide. It uses a DMSP-Iyase, encoded by dddL, the promoter-distal gene of a three-gene operon, acuR-acul-dddL. DMSP-cleavage and DMS production were previously shown to be slightly induced by pre-growth of cells with the substrate, DMSP. However, it was found that the catabolite acrylate was more effective and that the apparent the induction by DMSP requires its conversion to this bona fide eo-inducer. This unusual example of product-mediated regulation is controlled by the promoter-proximal gene acuR, which encodes a TetR-type transcriptional repressor. AcuR represses acuR-acul- dddL in the absence of acrylate, likely by binding to a palindromic eis-acting operator overlapping the acuR promoter. Unusually for bacteria, the acuk-acul-dddl: mRNA transcript is leaderless; it lacks a ribosome binding site and has no 5'-untranslated region. The AcuR repressor was translated at a lower level compared to the downstream genes, and there is an absolute requirement for 5'-terminal AUO-initiation codon of acuR. This regulatory unit may be widespread in bacteria, since several taxonomic ally distinct lineages have adjacent acuR-acul-like genes. These homologues are predicted to have no 5'-untranslated region or ribosome binding sites, and their predicted operators resemble those of Rhodobacter sphaeroides acuR-acul-dddL. The AcuI polypeptide, a medium chain oxidoreductase, is predicted to be indirectly involved in the catabolism of acrylate, since Acul mutant cells were far more sensitive to the toxic effects of this molecule, yet could still convert acrylate to CO2, albeit at lower levels than the wild type. Interestingly, acul-like genes are closely linked to a wide range of other genes involved in DMSP catabolism, in a range of bacterial types. A model for the possible evolution of the acuR-acul-dddL operon, and the possible function of its gene products is presented.
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