A DNA-dependent RNA polymerase was isolated from C. acidophila, a thermophilic bacterium living in acidic hot springs at temperatures ranging from 63-89.degree. C. The enzyme was purified 180-fold and is composed of 5 subunits having the following MWs: a = 127,000, b = 120,000, c = 72,000, d = 65,000 and e = 38,000. The enzyme is activated by Mn2+ and Mg2+ and exhibits optimal activity in the presence of 0.5 mM Mn2+. The activity depends on ionic strength, with a maximum at 0.25 M KCl, and exhibits a pH optimum at 7.8 in the presence of Tris-HCl buffer. The enzyme shows a high degree of thermophilicity, its temperature optimum being 80.degree. C in the in vitro assay. The thermophilicity of C. acidophila RNA polymerase allows studies on enzyme-template interactions to be performed in a temperature range where many templates are close to their Tm [melting point].