PHOTOSENSITIZED REACTION OF POLYRIBONUCLEOTIDES. I. EFFECTS ON THEIR SUSCEPTIBILITY TO ENZYME DIGESTION AND THEIR ABILITY TO ACT AS SYNTHETIC MESSENGERS

Mutations induced by methylene blue plus light in single-stranded M13mp2

Reactive oxygen species are generated by a variety of cellular processes. These endogenously generated, reactive intermediates produce a multiplicity of DNA alterations and mutations and have been implicated in the pathogenesis of several human diseases. We report here that treatment of single-stranded M13mp2 bacteriophage DNA with methylene blue and white light generates increased levels of 8-hydroxydeoxyguanosine and that mutagenesis is both highly specific and dependent on the SOS response. Lesions produced block the progression of DNA synthesis one base preceding template guanines. In SOS-induced Escherichia coli, 97% of all methylene blue-induced mutations in the lacZ alpha gene of M13mp2 DNA are single-base substitutions opposite template guanines. The most frequent mutations are G----C transversions. The G----T transversions expected from the presence of 8-hydroxydeoxyguanosine in the template strand occur, but at a lower frequency. Sequence data together with SOS dependency and the presence of replication blockage demonstrate that while 8-hydroxydeoxyguanosine may serve as an important marker to monitor oxygen-induced DNA damage in humans, it does not account for either the observed blockage to replication or the mutagenesis by methylene blue plus light in SOS-induced E. coli. Instead, an as yet unidentified lesion generated by active oxygen species is a more potent mutagenic event.

Escherichia coli Fpg protein and UvrABC endonuclease repair DNA damage induced by methylene blue plus visible light in vivo and in vitro

pBR322 plasmid DNA was treated with methylene blue plus visible light (MB-light) and tested for transformation efficiency in Escherichia coli mutants defective in either formamidopyrimidine-DNA glycosylase (Fpg protein) and/or UvrABC endonuclease. The survival of pBR322 DNA treated with MB-light was not significantly reduced when transformed into either fpg-1 or uvrA single mutants compared with that in the wild-type strain. In contrast, the survival of MB-light-treated pBR322 DNA was greatly reduced in the fpg-1 uvrA double mutant. The synergistic effect of these two mutations was not observed in transformation experiments using pBR322 DNA treated with methyl methanesulfonate, UV light at 254 nm, or ionizing radiation. In vitro experiments showed that MB-light-treated pBR322 DNA is a substrate for the Fpg protein and UvrABC endonuclease. The number of sites sensitive to cleavage by either Fpg protein or UvrABC endonuclease was 10-fold greater than the number of apurinic-apyrimidinic sites indicated as Nfo protein (endonuclease IR)-sensitive sites. Seven Fpg protein-sensitive sites per PBR322 molecule were required to produce a lethal hit when transformed into the uvrA fpg-1 mutant. These results suggest that MB-light induces DNA base modifications which are lethal and that these modifications are repaired by Fpg protein and UvrABC endonuclease in vivo and in vitro. Therefore, one of the physiological functions of Fpg protein might be to repair DNA base damage induced by photosensitizers and light.

Methylene blue plus light mediates 8-hydroxyguanine formation in DNA

Exposure to methylene blue (MB) plus light mediates formation of large levels of 8-hydroxyguanine in DNA. The amount of 8-hydroxy-2'-deoxyguanosine (8-OHdG) present in DNA increased as the amount of MB concentration increased throughout the 2 to 200 microM range studied and was dependent on light exposure. As the time of light exposure increased so did the 8-OHdG content to levels of about 750 8-OHdG/10(5) deoxyguanosine after 15 min of light exposure when MB was at 20 microM. Even though previous research has demonstrated that hydroxyl free radicals formed from a variety of sources mediate 8-OHdG formation in DNA, inclusion of mannitol, superoxide dismutase, catalase, and desferal in the MB plus light experiments demonstrated that these scavengers of oxygen free radical intermediates or precursors caused either no change or an increase in the 8-OHdG content of DNA exposed to MB plus light. These results appear to rule out the direct role of oxygen free radical intermediates in the primary events involved in the MB plus light mediated formation of 8-OHdG in DNA. Oxygen was essential to cause MB plus light mediated 8-OHdG formation in DNA. It was noted that when the reaction was carried out where the deuterium oxide content had been increased to 100%, the amount of 8-OHdG formed in DNA increased about threefold over that observed when comparable reactions were carried out in pure H2O. Use of the singlet oxygen scavenger 2,5-dimethylfuran has yielded variable results on the MB plus light mediated formation of 8-OHdG in DNA. The data taken collectively clearly indicate that MB plus light mediates 8-OHdG formation in DNA. The D2O data and the requirement for oxygen suggest that singlet oxygen may be an intermediate.

Structure and function of Escherichia coli formylmethionine transfer RNA: loss of methionine acceptor activity by modification of a specific guanosine residue in the acceptor stem of formylmethionine transfer RNA from Escherichia coli

The structural requirements of E. coli formylmethionine tRNA for aminoacylation have been examined by chemical modification of the tRNA, followed by separation of the modified molecules into active and inactive components. Photooxidation of tRNA(fMet) at 50 degrees in the presence of methylene blue results in modification of two guanosine (G) residues in the acceptor stem, at positions no. 2 and no. 71 from the 5'-phosphate terminus. Both of these modifications are present in inactive molecules, but only the G residue at position no. 2 is modified in the acceptor stem of active molecules. Loss of methionine acceptance occurs with first-order kinetics, indicating that inactivation by modification of G residue no. 71 is independent of any other modifications taking place under these conditions. The presence of a modified G residue at position no. 2 in the acceptor stem of active photooxidized molecules shows that disruption of normal base-pairing in this region is not sufficient to inactivate tRNA(fMet). These data indicate that the inactivating modification at position no. 71 is lethal due to a specific alteration in the nucleotide base, rather than simply as a result of breaking a hydrogen-bonded base pair in the acceptor stem.