Mapping of a contact for the RNA 3′ terminus in the largest subunit of RNA polymerase

Stalled elongation complexes of Escherichia coli RNA polymerase were prepared carrying the photocross-linkable 8-azido derivative of adenine at the 3′-terminus of the nascent RNA chain. Ultraviolet irradiation of such complexes resulted in the cross-linking of radiolabeled RNA exclusively to the β′ subunit of RNA polymerase. The adduct was mapped between Met⁹³² and Trp¹⁰²⁰ in the linear sequence of the β′ polypeptide using specific chemical degradation of the cross-linked species.