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J. Biol. Chem., Vol. 255, Issue 20, 9556-9563, Oct, 1980

Role of the sigma subunit of Escherichia coli RNA polymerase in initiation. I. Characterization of core enzyme open complexes

UM Hansen and WR McClure

Open complexes of Escherichia coli RNA polymerase core enzyme with a poly[d(A-T)]-poly[d(A-T)]template have been characterized and compared with the previously characterized holoenzyme open complexes on the same template (Hansen, U. M., and McClure, W. R. (1979) J. Biol. Chem. 254, 5713-5717). The open complexes were monitored by the abortive initiation synthesis of the dinucleotide pApU, which is catalyzed by both enzymes. The major differences between the two complexes were: 1) the Michaelis constant for UTP was 60 times higher for core enzyme than for holoenzyme, 2) the intrinsic binding constant of core enzyme to the DNA was 3 orders of magnitude less than that of holoenzyme, and 3) cooperative binding of 2 core units was required for activity. Thus, the presence of the sigma subunit significantly altered the nature and extent of open complex formation. The rate of formation of the open complexes, however, was rapid for both enzymes. Rifampicin is shown to have a slight stimulatory effect on the extent of open complex formation of the core enzyme.
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