Gram- symbiotic bacteria (called X-bacteria), present in the xD strain of Amoeba proteus as required cell components, contain a large amount of a 67-kDa protein, a GroEL analog. The complete nucleotide (nt) sequence of the groEx operon of X-bacteria has been determined and it has a high degree of nt identity with those of other bacterial groE operons. The groELx gene is expressed in transformed Escherichia coli and has a novel and potent promoter (P2) in addition to the heat-shock consensus promoter (P1). This is shown by the production of GroELx in Escherichia coli transformed with modified DNA clones lacking P1 and by an enhanced production of a GroELx::beta-galactosidase fusion protein when a portion of groEx containing P2 is linked to the lacZ gene. Primer-extension analyses revealed the presence of possible P2 sequences within the open reading frame of the groESx gene. It is suggested that the presence of a potent P2 in the X-bacterial gene is an adaptation for the endosymbiotic bacteria to survive within a potentially hostile intracellular environment.