In Streptomyces coelicolor A3(2), synthesis of the groES, groES-groEL1 and groEL2 transcripts is induced either by heat shock or by undefined physiological stress signals present at a certain stage of growth. Under all conditions tested, transcription of groES and groES-groEL1 originated from a unique start site upstream of groES, whereas transcription of groEL2 originated from a unique site upstream of groEL2. RNA polymerase isolated either from heat-shocked or control mycelia allowed in vitro transcription from the P1 promoter of groES/EL1 and the P2 promoter of groEL2. The fact that these two RNA polymerase preparations both initiated transcription with equal efficiency from the same sites suggested that a heat shock-specific sigma factor is not responsible for the temperature-induced transcription of groE genes. Instead, regulation of these genes from vegetative-type promoters may be effected by a DNA-binding protein observed in gel retardation assays, which recognizes a motif found in the groE and dnaK promoter regions of many prokaryotic genes.