The effects of covalent additions of a psoralen on transcription byE. coliRNA polymerase

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Abstract
Synthetic DNA substrates containing a site-specifically engineered psoralen monoadduct or diadduct were used to characterize the response of the E. coli RNA polymerase elongation complex to these lesions. The psoralen derivative HMT (4′-hydroxymethyl-4, 5′, 8-trimcthylpsoralen) was site specifically placed into two synthetic double-stranded DNA fragments each of which contained an E. coli RNA polymerase promoter at one end. The HMT molecule was attached to the middle of the DNA fragments as either a furan-side monoadduct or an interstrand diadduct Transcription off the HMT crosslinked DNA templates showed that E. coli RNA polymerase terminated at the HMT diadduct site, L e., one nucleotide before the modified thymidine residue on the template strand. The furan-side monoadduct when on the template strand also blocked transcription by the polymerase. However, no effect on transcription was observed when the monoadduct was located on the non-template strand.