A two-component regulatory system controls autoregulated serpin expression in bifidobacterium breve UCC2003

This work reports on the identification and molecular characterization of a two-component regulatory system (2CRS), encoded by serRK, which is believed to control the expression of the ser₂₀₀₃ locus in Bifidobacterium breve UCC2003. The ser₂₀₀₃ locus consists of two genes, Bbr_1319 (sagA) and Bbr_1320 (serU), which are predicted to encode a hypothetical membrane-associated protein and a serpin-like protein, respectively. The response regulator SerR was shown to bind to the promoter region of ser₂₀₀₃, and the probable recognition sequence of SerR was determined by a combinatorial approach of in vitro site-directed mutagenesis coupled to transcriptional fusion and electrophoretic mobility shift assays (EMSAs). The importance of the serRK 2CRS in the response of B. breve to protease-mediated induction was confirmed by generating a B. breve serR insertion mutant, which was shown to exhibit altered ser₂₀₀₃ transcriptional induction patterns compared to the parent strain, UCC2003. Interestingly, the analysis of a B. breve serU mutant revealed that the SerRK signaling pathway appears to include a SerU-dependent autoregulatory loop.