Abstract.During the emergence of the eukaryotic line and the three classes of DNA‐dependent RNA polymerases with their specific transcriptional tasks, the subunit structure of these enzymes became more complex when compared with the prokaryotic enzyme. It is proposed, especially for the RNA polymerase I from higher plants, that the increase in the complexity of subunit structure during evolution was associated with an increase in the degree of co‐operativity of substrate binding sites. This multiplicity of substrate binding sites could be useful for fitting and proper base‐pairing between the incoming substrate molecules and the DNA tem