Analysis of the mutations induced in the E. coli lac Z gene by a psoralen analog

A psoralen in which intracyclic oxygen atoms were replaced by sulfur (7H-thieno [3,2-g]-[1]-[benzothiopyran-7-one) [PSO(S-S)]) was recently synthesized and its photobiological properties were investigated. M13mp19 DNA photosensitization mediated by PSO (S-S) followed by transfection into competent E. coli gave rise to a very low phage progeny showing the high aptitude of this compound to modify DNA. In order to characterize the role of oxidative damages in the photosensitized reaction mediated by PSO(S-S), plasmid bearing the gene encoding the formamidopyrimidine DNA glycosylase (Fpg) under the control of the inducible lac Z promoter was transfected in E. coli. Overexpression of Fpg was induced by addition of isopropyl-beta-D-thio-galactopyranoside (IPTG) to the cells and monitored by western blot analysis. Fpg overexpression did not influence the rate of M13mp19 DNA photoinactivation by PSO(S-S) neither the mutation frequency measured by the expression of beta-galactosidase encoded by the lac Z gene beared by M13mp19. Analysis of the mutation patterns recorded with or without Fpg overexpression showed that several G to T transversions due to oxidative damages were repaired by Fpg. These data show that oxidative DNA damages generated during PSO(S-S) photosensitization have only limited biological implications measured in terms of DNA photoinactivation.