Showing papers by "Gagik G. Gurzadyan published in 1994"

Damage to uracil- and adenine-containing bases, nucleosides, nucleotides and polynucleotides: quantum yields on irradiation at 193 and 254 nm.

Abstract: Photoreactions, such as base release and decomposition of the base moiety, induced by either 20 ns laser pulses at 193 nm or continuous 254 nm irradiation, were studied for a series of uracil and adenine derivatives in neutral aqueous solution. The quantum yield of chromophore loss (phi cl) depends significantly on the nature of the nucleic acid constituent and the saturating gas (Ar, N2O or O2). In the case of polynucleotides the destruction of nucleotides was measured by high-performance liquid chromatography after hydrolysis; the quantum yields (phi dn) are comparable to those of chromophore loss or larger. The phi cl and phi dn of 0.04-0.1 for poly(U) and poly(dU), obtained for both wavelengths of irradiation, are due to processes originating from the lowest excited singlet state, i.e. formation of photohydrates and photodimers, and a second part from photoionization using lambda irr = 193 nm. Irradiation at 193 nm effectively splits pyrimidine dimers and thus reverts them into monomers. The quantum yield for release of undamaged bases (phi br) from nucleosides, nucleotides and polynucleotides upon irradiation at 254 nm is typically phi br = (0.1-1) x 10(-4). Breakage of the N-glycosidic bond is significantly more efficient for lambda irr = 193 nm, e.g. phi br = 1.1 x 10(-3), 0.8 x 10(-3), 4.3 x 10(-3) and 0.5 x 10(-3) for poly(A), poly(dA), poly(U) and poly(dU) in Ar-saturated solution, respectively. Enhanced phi values for lambda irr = 193 nm, essentially for adenine and its derivatives, are caused by photoprocesses that are initiated by photoionization.

Are enzymatically produced single-strand breaks involved in UV-induced inactivation of plasmid DNA?

Abstract: pBR322 plasmid DNA was exposed to 254 nm UV radiation and examined for enzymatically produced single-strand break (sbb) and double-strand break (dsb) formation by treatment with an extract containing the proteins of Escherichia coli (AB1157 (uvrA+ recA+) and AB2480 (uvrA- recA-)). Enzymatic conversion of the 254 nm-induced lesions into ssbs on treatment with an extract from AB1157 was observed, but not conversion into dsbs. The rate of enzymatic ssb formation in the AB1157 extract is initially higher than in the AB2480 extract, the sbb formation levels off leading to plateau values with increasing incubation time. The rate of ssb formation in the AB2480 extract is initially lower, but does not level off, and the ssb yield becomes larger at longer incubation times than that with the AB1157 extract. The biological inactivation of the plasmids was measured as a function of 254 nm fluence by transformation of E. coli AB1157 and AB2480. Inactivation with AB2480 is mainly due to a single photoproduct, a cyclobutane-type pyrimidine dimer, per DNA molecule. Inactivation with AB1157 occurs with a quantum yield which is virtually identical with that of the plateau values of enzymatic ssb formation, as measured by incubation in the AB1157 extract. A possible interpretation is that the formation of irreparable ssbs is the lethal step in the sequence of events leading to inactivation of plasmid DNA in the repair wild-type strain. The quantum yield of inactivation is 10-20 times smaller for transformation of AB1157 than for AB2480, indicating that enzymatic repair of photolesions of the plasmid occurs in AB1157.