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The Yersinia low-Ca2+ response (LCR) is a regulatory response in which a set of plasmid-borne operons is transcriptionally regulated at 37 degrees C in response to the presence or absence of mM concentrations of Ca2+. LCR-regulated operons encode secreted proteins with regulatory and virulence roles as well as non-secreted regulatory proteins and components of the secretion machinery. Downregulation by Ca2+ is imposed by a signalling cascade that includes secreted proteins and possibly also components of the secretion system and is hypothesized to act on membrane-bound inductive components. An important role in LCR induction is played by LcrD, an inner-membrane protein with homologues in several virulence-associated and flagella assembly-related systems in diverse bacterial species. The mechanism of signal transduction in response to Ca2+ is not known, and the proteins that bind DNA to downregulate transcription have not been identified.