Malondialdehyde adducts in DNA arrest transcription by T7 RNA polymerase and mammalian RNA polymerase II

Malondialdehyde, a genotoxic byproduct of lipid peroxidation, reacts with guanine in DNA to form pyrimido[1,2-alpha]purin-10(3H)-one (M(1)dG), the first endogenous DNA lesion found to be a target of nucleotide excision repair enzymes. A subpathway of nucleotide excision repair, transcription-coupled repair, is thought to occur when RNA polymerase (RNAP) is arrested at damage in transcribed DNA strands and might function for efficient removal of M(1)dG in active genes. Results presented here show that M1dG and its stable, exocyclic analog 1,N-2-propanodeoxyguanine (PdG), arrest translocation of T7 RNAP and mammalian RNAIPII when located in the transcribed strand of a DNA template. M1dG paired with thymine is exocyclic and poses a stronger block to transcription than the acyclic N-2-(3-oxo-1-propenyl)-dG, formed upon cytosine-catalyzed opening of M1dG in duplex DNA. PdG is a complete block to RNAPII regardless of base pairing. The elongation factor TFIIS (SII) induces reversal and RNA transcript cleavage by RNAPII arrested at PdG. Thus, arrested RNAPII complexes may be stable at M1dG in cells and may resume transcription once the offending adduct is removed. The conclusion from this work is that malondialclehyde adducts in the transcribed strand of expressed genes are strong blocks to RNAPs and are targets for cellular transcription-coupled repair. If so, then M1dG, already known to be highly mutagenic in human cells, also may contribute to apoptosis in the developing tissues of individuals with Cockayne's syndrome, a hereditary disorder characterized by transcription-coupled repair deficiency.