Résumé
Formation of a transcriptionally competent open complex is a
highly regulated multistep process involving at least two intermediates.
The rate of formation and stability of the intermediate complexes often
determine promoter strength. However, the detailed mechanism of
formation of the open complex and the high resolution structures
of these intermediates are not known. In this study the structures
of the open and intermediate complexes formed on the lacUV5
promoter by Escherichia coli RNA polymerase were
analyzed using ‘zero length’ DNA–protein
cross-linking. In both the open and the intermediate complexes the
core subunits (β′ and β) interact
with lacUV5 DNA in a similar way, forming DNA–protein
contacts flanking the initiation site. At the same time, the recognition
(σ70) subunit closely interacts
with the promoter only in the open complex. In combination with
our previous results, the data suggest that during promoter recognition
contacts of the σ subunit with core
RNA polymerase and promoter DNA are rearranged in concert. These rearrangements
constitute a landmark of transition from the intermediate to the
open complex, identifying the σ subunit
as a key player directing formation of the open complex.