Significance and context Exposure of cells to a sudden rise in
temperature leads to the protective 'heat-shock response' that
induces the synthesis of a group of highly-specialized heat-
shock proteins (HSPs). Many HSPs are molecular chaperones (for
example, the bacterial DnaK, GroEL) or ATP-dependent proteases
(for example Lon and Clp) involved in the modulation of protein
folding, assembly, repair and degradation. In response to heat
shock, bacteria have at least two main transcriptional control
strategies. One is the transient up-regulation of heat-shock-
gene expression via the transcription factor σ32 (RpoH). The
other is the induction of transcription of heat-shock genes by
removal of repressor proteins bound to specific DNA regions,
such as the CIRCE repeat (controlling inverted repeat of
chaperone expression). In various rhizobial species, a further
regulatory element that controls at least five heat-shock
operons consists of a conserved 100 base-pair DNA segment in the
5'-untranslated region, between t