UV-LIGHT INDUCES IS10 TRANSPOSITION IN ESCHERICHIA-COLI

A new mutagenesis assay system based on the phage 434 cl gene carried on a lo low-copy number plasmid was used to investigate the effect of UV light on intermolecular transposition of IS10. Inactivation of the target gene by IS10 insertion Tvas detected by the expression of the t et gene from the phage 434 PK promoter, followed by Southern blot anal ysis of plasmids isolated from Tet(R) colonies. UV irradiation of cell s harboring the target plasmid and a donor plasmid carrying an IS10 el ement led to an increase of up to 28-fold in IS10 transposition. Each UV-induced transposition of IS10 was accompanied by fusion of the dono r and acceptor plasmid into a cointegrate structure, due to coupled ho mologous recombination at the insertion site, similar to the situation in spontaneous IS10 transposition. UV radiation also induced transpos ition of IS10 from the chromosome to the target plasmid, leading almos t exclusively to the integration of the target plasmid into the chromo some. UV induction of IS10 transposition did not depend on the umuC an d uvrA gene product, but it was not observed in lexA3 and Delta recA s trains, indicating that the SOS stress response is involved in regulat ing UV-induced transposition. IS10 transposition, known to increase th e fitness of Escherichia coli, may hale been recruited under the SOS r esponse to assist in increasing cell survival under hostile environmen tal conditions. To our knowledge, this is the first report on the indu ction of transposition by a DNA-damaging agent and the SOS stress resp onse in bacteria.