The α subunit of Escherichia coli RNA polymerase plays a major role in the subunit assembly. Carboxy-terminal deletion derivatives lacking 73 or 94 amino acid residues were assembled in vitro into enzyme molecules. Core enzymes consisting of these C-terminal-truncated α subunits were as active in RNA synthesis as native core enzyme. By the addition of σ70 subunit, these mutant enzymes initiated transcription from certain promoters. The mutant RNA polymerases, however, did not show cAMP-CRP activated transcription. These results demonstrate that the N-terminal region of the α subunit is involved in the formation of active enzyme molecule, while the C-terminal region plays an essential role in response to transcription activation by cAMP-CRP.