DnaK dependence of the mycobacterial stress-responsive regulator HspR is mediated through its hydrophobic C-terminal tail.

HspR is a repressor known to control expression of heat shock operons in a number of Eubacteria. In mycobacteria and in several other actinobacteria, this protein is synthesized from the dnaKJE-hspR operon. Previous investigations revealed that HspR binds to the operon promoter, thereby controlling its expression in an autoregulatory manner. DnaK, which is a product of the same operon, further aids this autoregulatory process by stimulating the operator binding activity of HspR. The molecular mechanism by which DnaK assists HspR in executing its function is not clearly understood. In this study, it has been shown that DnaK can augment DNA binding activity of HspR by two mechanisms: (i) DnaK can restore the activity of completely denatured HspR by forming a complex with it, and (ii) DnaK can prevent thermal instability of HspR renatured by other means. Unlike the first mechanism, the latter function does not involve complex formation. The C-terminal hydrophobic tail of HspR was found to play a significant role in determining its thermal stability and DnaK dependence properties. A deletion mutant in which this region is removed does not respond to thermal stress and functions independent of DnaK. The hydrophobic C-terminal tails of HspRs of Mycobacterium tuberculosis and related Actinomycetales therefore may have evolved to make these HspRs more sensitive to thermal stress and, at the same time, subject to regulation by DnaK.