Expression of the lsrA gene from Rahnella aquatilis, encoding levansucrase, is tightly regulated by the growth phase of the host cell; low-level expression was observed in the early phase of cell growth, but expression was significantly stimulated in the late phase. Northern blot analysis revealed that regulation occurred at the level of transcription. The promoter region was identified by primer extension analysis. Two opposite genetic elements that participate in the regulation of lsrA expression were identified upstream of the lsrA gene: the lsrS gene and the lsrR region. The lsrS gene encodes a protein consisting of 70 amino acid residues (M(r), 8,075), which positively activated lsrA expression approximately 20-fold in a growth phase-dependent fashion. The cis-acting lsrR region, which repressed lsrA expression about 10-fold, was further narrowed to two DNA regions by deletion analysis. The concerted action of two opposite regulatory functions resulted in the growth phase-dependent activation of gene expression in Escherichia coli independent of the stationary sigma factor sigma(S).