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Within the latter decade, sigma subunits of bacterial-type RNA polymerases were found in eukaryote cells. In the higher plants and algae, these subunits determine the promoter specificity of the chloroplast multisubunit RNA polymerase. In the higher plants, sigma subunits are encoded by the family of nuclear Sig genes comprising 5–6 genes. Several conclusions as to the evolution of this gene family were deduced from the comparison of the deduced amino acid sequences and the sites of intron location.