The Effect of a Drug-resistance Factor on Recombination and Repair of DNA in Escherichia coli K12

Abstract

The presence in recipient strains of Escherichia coli K 1 2 of the plasmid R46 greatly reduced the yield of recombinants from crosses with several Hfr strains and virtually abolished the formation of recombinants by P I transduction without, however, significantly affecting the transfer of the F prime from a strain carrying Fgal. The R46 plasmid had paradoxical effects on mutability: it appeared to enhance the yield of mutants following irradiation with ultraviolet light but it reduced the number of mutants detectable in unirradiated cultures. The effects of this plasmid on ultraviolet light survival of the wild type and several mutants defective for recombination and repair have been measured and the results, in the main, confirm similar observations by Tweats et al. (1976). Not only is the survival of the strain harbouring R46 greater than that of the parent strain in all the cases studied, but the survival of ultraviolet irradiated bacteriophage h is also greater. I N T R O D U C T l O NA number of enzymic mechanisms have been suggested for the processes of repair and recombination of DNA in the bacterium Escherichia coli K 1 2 (for reviews see Grossman eta/., 1975;Clark, 1973). In addition to the chromosomal gene functions involved, identified by rec, uvr, lex and sbc mutations, a number of extrachromosomal elements appear to be able to influence the repair and recombination of DNA. The increased survival after ultraviolet (u.v.) irradiation of strains carrying ColI compared with their non-colicinogenic parent strains was first described by Howarth (1965) and the effects of other bacterial plasmids which confer on their hosts an increased resistance to DNA damaging agents have been discussed (Drabble & Stocker, 1968;Marsh & Smith, 1969;Siccardi, 1969). With the increased survival there is a concomitant increase in the induced mutation rate (Krishnapillai, I 975) and in Salmonella typhimurium the spontaneous mutation rate is also increased (Mortelmans & Stocker, personal communication). However, the mechanism(s) responsible for these phenomena are unknown although one plasmid, R-Utrecht, has been shown to code for a new DNA polymerase (MacPhee, 1974). As certain of the gene products that are involved in the repair of DNA are also required for recombination, the effect of R46 (= R1818 = R-Brighton), a plasmid which increases the resistance of S. typhimurium to U.V. light, was investigated. In this paper the effects of this plasmid on U.V. survival and recombination in wild-type strains and in certain mutant strains are reported. With the exception of the results relating to recA strains, the data confirm and extend the observations concerned with the U.V. survival of R46+ strains reported by Tweats et al.

LexA-independent expression of a mutant mucAB operon

“…Despite numerous attempts to recombine this mutation into a mucA+B+ derivative, we have not been able to detect recombination between the 119-bp cloned pGW16 fragment and a mucA+B+ pKM101 derivative under conditions in which recombination frequencies of 10-5 or greater would have been detected. The reason for this lack of recombination is as yet undetermined but may be related to the observations of several investigators that the presence of pKM101 or its parent, R46, in a cell results in a decreased frequency of conjugational recombination (2,24).…”