As an attempt to identify the maturation pathway of Escherichia coli RNA polymerase, the catalytic properties of core enzyme reactivated in the absence of maturation-promoting factors (sigma subunit or DNA) (that is, of self-reactivated core enzyme) were compared with those of native core enzyme. Differences have been found in the intrinsic activities such as in the template specificity, Km value of DNA template for the polymerase, activation energy for RNA synthesis, and increment of enzyme activity by sigma subunit. These observations imply that the transcription initiation by self-reactivated core enzyme is inaccurate and, therefore, more strict conditions including the presence of maturation-promoting factors are required for premature core be activated to the genuine function with the transcription specificity of native core enzyme.