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~f~n ~'-.=T.~ 2~6 (1f~3) 255-265 253 Participation of the SOS system in producing deletions in E. coli plasmids Elias Balbinder, Bryan Coil, Jeff Hutehinson, Anthony S. Bianehi *, Tobi Groman *, Kevin A. Wheeler * and Matt Meyer * Department of Bi~chnn~try, Biaphy~cs and Genetfcs, and Colorado Cancer Canter, Univemity of Colorado Health Sciences Center, 4200 F.. N'mth Avenue, P.O. Box B-121, Denner, CO 80262, USA (Received 7 January 1992) (Revision received 22 September 1992) (Aecel~ted 25 September 1992) Keywords: SOS system, participation; Escherichla coli plasmids; Plasmids, E. coli Summar~ The participation of the SOS response in the deletion of palindromic and non-palindromic inserts of about 66 and 100 bp cloned within the EcoRl site of the ehloramphenicol aeetyl transferase (cat) gene of plasmid pBR325 was tested after introducing the derived plasmids into strains containing different combinations of/erA, recA and umuC alleles and the anxotrophic mutation trpE65. This allowed for a comparison of deletion frequency in the plasmids, measured as the reversion of ehloramphenieol sensitivity to resistance (CmS~ Cmr), to point-mutation frequency measured from the reversion of trpE65 to ttTptophan "independence (Trp---* Trp+). We found that the spontaneous deletion frequency of palindromic inserts was increased by the overproduction of activated ReeA * and UmuC+ in lexA (De0 backgrounds but the deletion of the non-palindromic inserts was unaltered. Overproduction of ReeA+ had no significant effect on deletion incidence but it did increase Trp---* Trp+ reversions. The SOS stimulation of palindrome deletions paralelled the SOS mutator effect of certain recA and urnuC alleles on Trp--~ Trp+ reversions, suggesting that some form of SOS processing was responsible for the observed increases. The results further suggest that the SOS effect on deletions depends on the distinction between palindromy vs. non-palindromy, rather than on the sizes or sequences of the inserts or those of the terminal homologies bracketing them. Spontaneous deletions in prokaryotie replicons can occur through a variety of mechanisms and inehide the resolution of unstable transient inter- * Summer Student Fellows of the Colorado Cancer Center. CorreslXmdence: Dr. Elias Balbinder, Department of Bio- chemistry, Bioph~'sies and Genetics and Col~rado Cancer Center, University of Colorado Health Sciences Center, 4200 F_.. lqinth Avenue, P.O. Box B-121, Denver, CO 8~262, USA. mediates formed in the normal course of DNA metabolism (Albertini et al., 1982; Ripley and Gliekman, 1983), intermoleeular reeombinatlon events (Dianov et al., 1991; Mazin et al., 1991; Singer and Westlye, 1988) or mistakes in the movement of transposable elements (Kleckner et" al., 1979). A large number of deletions occur between direct terminal repeats, which often flank inverted repeats (palindromes). Actor ,ding to mis- alignement mutagenesis models, these sequences can transiently stabilize unstable intermediates such as hairpins or erueiforms which may form on 40000085