Expression of Bacillus subtilis purine (purE) operon is a subject of double negative control involving repressor protein PurR and a transcription terminator located in the operon leader region. We have performed site-directed mutagenesis of the specific motives, which are involved in formation of alternative hairpin structures, one of which produces transcription termination at the leader region ofpurEoperon. In vivo and in vitro analyses of the generated mutants have shown that purine bases, guanine and hypoxantine, serve as effector metabolites capable of increasing stability of terminating hairpin within the leader mRNA. Therefore, the leader RNA of purE operon serves as a sensor towards these metabolites and a riboswitch that provides premature termination of the operon transcription. The synergistic effect of the PurR repressor protein and a transcription terminator located at the leader region on the expression of purE operon was also revealed.