Distinct forms of DPD constitute the interconverting molecules family generally called autoinducer 2 (AI-2). The binding of DPD to LsrR inactivates LsrR, but phosphorylated DPD appears to specifically inactivate the action of LsrR on the expression of genes for autoinducer 2 uptake and processing (lsrACDBFG-tam operon) and lsrR itself [5]. On the other hand, it was suggested that dihydroxyacaetone phosphate inhibits binding of DPD to LsrR [9]. The structure of the LsrR protein has been analyzed in crystallography and mutational analysis studies, where it was observed that it contains two main domains, the DNA-binding domain located in the N terminus and a ligand-binding domain around the C terminus [4]. This protein binds to DNA as a tetramer to regulate transcription; however, in the presence of phosphorylated autoinducer 2 (AI-2) or ADP-P, the tetramer dissociates into dimers, and the interaction of LsrR with DNA is greatly reduced [4]. The structure of LsrR as a tetramer was determined via exclusion chromatography and cross-linking assays [4]. The crystal structure of the LsrR protein complexed with the native signal (phosphor-Al-2, D5P) was determined [10] D5P is converted into its hydrated form to create strong H-bonds with the carboxyl group of D243 [10] The lsrR gene is transcribed together with lsrK in an operon [3], and this last gene encodes a protein that phosphorylates to DPD [11]. The lsrRK operon is transcribed divergently from the lsrACDBFG-tam operon, and both operons are activated by CRP and repressed by LsrR [3]. The expression of lsrR was decreased in uvrY and csrA mutant strains [12] lsrR shows differential codon adaptation, resulting in differential translation efficiency signatures, in thermophilic microbes. It was therefore predicted to play a role in the heat shock response. An lsrR deletion mutant was shown to be more sensitive than wild-type specifically to heat shock, but not other stresses [13].