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i M~a£an P~-~ra~, 2~6 (1~3) 253-265 O 1S93 ETz~cr St_on-,- P~._-rs B.V. AII r~.~ts r~r,'~l ~'327-5107/93./S~5£'3 253 M'UT 052~5 Participation of the SOS system in producing deletions in E. coli plasmids Elias Balbinder, B~an Coil, Jeff Hutchinson, Anthony S. Bianchi *, Tobi Groman *, Kevin A. Wheeler * and Matt Meyer * Department of Biocheralstr~, BiophyMc.¢ and Geneac~, and Colorado Cancer C~. . te.r, ~Jnlversity of Colorado Health ~ienecs Center, 4200 E. lV-mth Avenue, P.O. Box B-121, Denver, CO 80262, USA (Rrz~ived 7 January 1992) (Revision received 22 September 1992) (Accepted 25 September 1992) Keywords: SOS system, paxticipation; Eschefichia coil plasmids; Flas .raids, Summary The participation of the SOS response in the deletion of palindromic and non.palindromic inserts o£ about 66 and 100 bp cloned within the EcoR1 site of the chloramphenicol acetyl transferase (cat) gene of plasmid pBR325 was tested after introducing the derived plasmids into strains containing different combinations of/exA, recJl and umuC alleles and the auxotrophic mutation trpE65. This allowed for a comparison of deletion frequency in the plasmids, measured as the revexsion of chloramphenicol sensitivity to resistance (CmS-~ Cmr), to po'mt-mutation frequency measured from the reversion of trpE65 to tryptophan independence (Trp-~ Trp +). We found that the spontaneous deletion frequency of palindromic inserts was increased by the overproduction of activated RecA * and UmuC+ in/exA (DeO backgrounds but the deletion of the non-palindromic inserts was unaltered. Overproduction of RecA+ 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 rec,4 and umuC 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 prokaryotic replicons can occur through a variety of mechanisms and include the resolution of unstable transient inter- * Summer Student Fellows of the Colorado Canee~ Center. Correspondence: Dr. Elias Balblnder, Department c~f Bio- chem~try, Biophy~cs aud Genetics and Colorado Cancer Center, University of Colorado Health Sciences Center, 4200 E. N'mth Avenue, P.O. Eex B-121, Denver, CO 89262, USA. mediates formed in the normal course of DNA metabolism (Albertini et al., 1982; Ripley and Glickman, 1983), intermoleeular recombination events (D[anov et al., 1991; Mazin et aL, 1991; Singer and Westlye, 1988) or mistakes in the- movement of transposable elements (Kleekner et al., 1979). A large number of deletions occur between direct terminal repeats, which often flank inverted repeats (palindromes). According to mis- alignement mutagenesis models, these sequences can trausientiy stabilize unstable intermediates such as hairpins or erueiforms which may form on 40000070