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.~,:~.~n Research, 29~ (1903) 193-209 © I~93 EIs~'i=r Sc~en=e PnhHsh~rs B.V. All rlghls reserved 0165-1218/93/$05.C~} 193 Multiple pathways of deletion formation in Esche~ichia coli Elias Balbinder Department of Biochemistry, Bh71~hys~cs and Genetics and Colorado Cancer Centf.r, University of Colorado Health Sciences Center, 4200 East 9th Auenue, P.O. Box B 121, Denver, CO 8026~ USA (Received 24 June 1992) (Revision received 29 December 1992) (Accepted 11 January 1993) Keywords: Deletions; Palindromes; SOS repair; Tnl0 bxcision; RecBC; Mutant selection for deletion enhancement Summary We are investigating the mechanisms for deletion formation through the use of mutants which alter deletion frequency together with well characterized systems for deletion detection. We report here on three mutations which were isolated for their ability to stimulate deletions in plasmid pMC874 (dli mutations). The mutation rec-2251 (formerly .known as dlil) is a new allele of recBCD, a group of genes coding for the polypeptide components of the major recombination enzyme complex in E. cell; the second one, dli2 may be a new allele of uvrD, which codes for DNA helicase II; a.nd the third one, dli3, has the phenotype of a mismatch repair mutation. Here we compare the effects of mutations in SOS-repair genes to those of the d/i mutations on three different deletion events: (a) the deletion of short (60-100-bp) palindromic and non-palindromic inserts in derivatives of plasmid pBR325; (b) larger (600-800-bp) deletions in plasmid pMC874; and (c) the excision of the Tnl0 transposon from chromoso- mal sites. Our results indicate that some form of SOS processigg stimulates the loss of palindromes but not non-palindromes in plasmid pBR325 derivatives, and that ReeA is necessary for UV-indueed excision of Tnl0 but this event is inhibited by UmuCD or its homolog MucAB. Each of the dli mutations showed unique effects on different classes of deletions. Mutation rec-2251 stimulated specifically deletions in pMC874 but had no effect on the deletion of non-palindxomes in pBR325, and reduced the incidence of the other deletion events tested including loss.of palindromic inserts in pBR325 as well as Tnl0 excision. Mutation d//2, on the other hand, stimulated all deletions tested t9 vaxlring extents, while d//3 did not affect markedly deletion formation in pBR325 plasmids but had a large stimulator/effect on both deletions in plasmid pMC874 and Tnl0 excision. These results reveal that (a) some SOS-repair functions participate in deletion formation, (b) mutations selected for altering the incidence of one class of deletions may have totally different effects on other deletion events, and (e) the differences in mutant Correspondence: Dr. Elias Balbinder, Department of Bio- chemistry, Biophysics and Genetics and Cclvrado Cancer Center. University of .Colorado Health Sciences Center, 4200 East 9th Avenue, P.O. Box B 121, Denver, CO 81}262, USA. 40000089