CosR is a repressor of compatible solute biosynthesis and transporter systems

Abstract

AbstractBacteria accumulate small, organic compounds, called compatible solutes, via uptake from the environment or biosynthesis from available precursors to maintain the turgor pressure of the cell in response to osmotic stress.Vibrio parahaemolyticushas biosynthesis pathways for the compatible solutes ectoine (ectABCasp_ect)and glycine betaine (betIBAproXWV), four betaine-carnitine-choline transporters (bcct1-bcct4) and a second ProU transporter (proVWX).Most of these systems are induced in high salt. CosR, a MarR-type regulator, which is divergently transcribed frombcct3, was previously shown to be a direct repressor ofectABCasp_ectinVibriospecies. In this study, we investigated the role of CosR in glycine betaine biosynthesis and compatible solute transporter gene regulation. Expression analyses demonstrated thatbetIBAproXWV,bcct1,bcct3, andproVWXare repressed in low salinity. Examination of an in-framecosRdeletion mutant shows induced expression of these systems in the mutant at low salinity compared to wild-type. DNA binding assays demonstrate that purified CosR binds directly to the regulatory region of each system. InEscherichia coliGFP reporter assays, we demonstrate that CosR directly represses transcription ofbetIBAproXWV,bcct3, andproVWX. Similar toV. harveyi, we showbetIBAproXWVis positively regulated by the LuxR homolog OpaR. Bioinformatics analysis demonstrates that CosR is widespread within the genus, present in over 50 species. In several species, thecosRhomolog was clustered with thebetIBAproXWVoperon, which again suggests the importance of this regulator in glycine betaine biosynthesis. Incidentally, in fourAliivibriospecies that contain ectoine biosynthesis genes, we identified another MarR-type regulator,ectR, clustered with these genes, which suggests the presence of a novel ectoine regulator. Homologs of EctR in this genomic context were present inA. fischeri, A. finisterrensis, A. sifiaeandA. wodanis.ImportanceVibrio parahaemolyticuscan accumulate compatible solutes via biosynthesis and transport, which allow the cell to survive in high salinity conditions. There is little need for compatible solutes under low salinity conditions, and biosynthesis and transporter systems are repressed. However, the mechanism of this repression is not fully elucidated. CosR plays a major role in the repression of multiple compatible solute systems inV. parahaemolyticusas a direct negative regulator of ectoine and glycine betaine biosynthesis systems and four transporters. Homology analysis suggests that CosR functions in this manner in many otherVibriospecies. InAliivibriospecies, we identified a new MarR family regulator EctR that clusters with the ectoine biosynthesis genes.