Heat-shock regulation was detected for three out of the five members of the groESL multigene family in Bradyrhizobium japonicum. The results uncovered the simultaneous presence of two distinct heat-shock control systems which so far have not been reported to co-exist in a single prokaryotic organism. The first system concerns groESL1 whose transcription is controlled in a sigma32-dependent manner similar to that known from work done with Escherichia coli. Heat-shock control of groESL4 is mediated by the second system, which is characterized by an inverted-repeat DNA structure originally described as a heat-shock regulatory element (CIRCE) in Bacillus subtilis. This element represses expression of groESL4 under non-stress conditions, as inferred from the increased expression of a groESL4'-'lacZ fusion suffering a 4 bp deletion within the CIRCE element. The two control systems clearly differ with respect to the temperature dependence and the kinetics of the heat-shock response, and they also respond differently to the stress signal elicited by incorporation of the amino acid analogue p-F-phenylalanine into cellular protein. Knock-out mutations in groEL4 resulted in an increased expression of groESL4, suggesting that repression via CIRCE depends, itself, upon the cellular level of GroEL4 protein.