Bacillus subtilis ypaA gene regulation mechanism by FMN riboswitch

We studied the regulation of the Bacillus subtilis ypaA gene-encoding riboflavin-transporter by FMN riboswitch. Using translational fusions of the leader region of wild-type ypaA gene with the lacZ-reporter gene in the leader region we showed that in vivo ypaA gene expression decreased more than 10-fold in the presence of endogenous FMN. Introduction of two nucleotide substitutions providing stabilization of the sequester hairpins results in almost complete repression of reporter gene expression. Using toeprint assay in vitro it has been shown that FMN presence inhibits the formation of the 30S initiation complexint the ypaA gene leader mRNA. Our results support the model of ypaA gene regulation whereby FMN binding with the ypaA gene leader sequence results in translation suppression through the sequestering of the SD-sequence.