Bacterial thermosensor protein : studies on the functions of HrcA protein as a transcriptional regulator for chaperone genes.

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In the heat-shock response of bacillary cells, HrcA repressor proteins negatively control the expression of the major heat-shock genes, groE and dnaK operons by binding the CIRCE (controlling inverted repeat of chaperone expression) element. The studies on two critical but yet unrevealed issues related to the structure and function of HrcA were performed mainly using the HrcA from obligate thermophile Bacillus thermoglucosidasius KP1006. These two critical issues are : (i) the identification of the region for HrcA to bind the CIRCE element, and (ii) whether HrcA can play the role of a thermosensor. We assigned the position of a helix-turn-helix (HTH) motif in B. thermoglucosidasius HrcA, which is one of the typical for DNA-binding proteins, and indicated that two residues in the HTH motif are crucial for the binding of HrcA to the CIRCE element. Furthermore, we compared the thermostability of the HrcA-CIRCE complex derived from Bacillus subtilis and B. thermoglucosidasius, which grow in vastly different ranges of temperature. The profiles of their HrcA-CIRCE complexes in thermostability were quite consistent with the difference in the growth temperature of B. thermoglucosidasius and B. subtilis and, thus, suggested that the HrcA can function as a thermosensor to sense temperature change in cells.