The c-di-GMP effector FleQ controls alginate production by repressing transcription of in .

Production of the exopolysaccharide alginate by , member of the family, is positively controlled by the second messenger c-di-GMP. This effect was solely attributed to the role of c-di-GMP in activating the alginate polymerase complex. In this study, the role of c-di-GMP in transcription, which encodes the key enzyme for alginate synthesis, was investigated. transcription correlated with artificially high or low levels of c-di-GMP. Moreover, FleQ, one of the best-characterized c-di-GMP effectors, was found to exert a negative effect on alginate production and transcription, as both increased in a Δ mutant relative to the wild-type strain or the Δ/+ complemented strain. Electrophoretic mobility shift assays (EMSAs) confirmed that FleQ directly binds to the regulatory region of , which was consistent with the presence of two FleQ binding sites in the vicinity of the RpoS-dependent promoter. In , c-di-GMP is essential for the expression of alginate C-5 epimerases (AlgE1-6), which are necessary for structuring the envelope of differentiated cells, known as cysts. However, FleQ was not involved in this regulation. Collectively, our results support a model in which transcription is under the positive control of c-di-GMP, while FleQ may only partially mediate this effect. In contrast, our study revealed a FleQ-independent regulatory mechanism for the control of encystment.