Increased Episomal Replication Accounts for the High Rate of Adaptive Mutation in recD Mutants of Escherichia coli

Corresponding author: Patricia L. Foster, S107, Boston University School of Public Health, Boston University School of Medicine, 715 Albany St., Boston, MA 02118-2394., pfoster{at}bu.edu (E-mail)

Communicating editor: R. MAURER

Adaptive mutation has been studied extensively in FC40, a strain of Escherichia coli that cannot metabolize lactose (Lac^-) because of a frameshift mutation affecting the lacZ gene on its episome. recD mutants of FC40, in which the exonuclease activity of RecBCD (ExoV) is abolished but its helicase activity is retained, have an increased rate of adaptive mutation. The results presented here show that, in several respects, adaptive mutation to Lac⁺ involves different mechanisms in recD mutant cells than in wild-type cells. About half of the apparent increase in the adaptive mutation rate of recD mutant cells is due to a RecA-dependent increase in episomal copy number and to growth of the Lac^- cells on the lactose plates. The remaining increase appears to be due to continued replication of the episome, with the extra copies being degraded or passed to recD⁺ recipients. In addition, the increase in adaptive mutation rate in recD mutant cells is (i) dependent on activities of the single-stranded exonucleases, RecJ and ExoI, which are not required for (in fact, slightly inhibit) adaptive mutation in wild-type cells, and (ii) enhanced by RecG, which opposes adaptive mutation in wild-type cells.

This article has been cited by other articles:

				J. D. Stumpf, A. R. Poteete, and P. L. Foster Amplification of lac Cannot Account for Adaptive Mutation to Lac+ in Escherichia coli J. Bacteriol., March 15, 2007; 189(6): 2291 - 2299. [Abstract] [Full Text] [PDF]

				P. L. Foster Adaptive Mutation in Escherichia coli J. Bacteriol., August 1, 2004; 186(15): 4846 - 4852. [Full Text] [PDF]

				P. L. Foster Rebuttal: Growth under Selection Stimulates Lac+ Reversion (Roth and Andersson) J. Bacteriol., August 1, 2004; 186(15): 4861 - 4861. [Full Text] [PDF]

				J. M. Budzik, W. A. Rosche, A. Rietsch, and G. A. O'Toole Isolation and Characterization of a Generalized Transducing Phage for Pseudomonas aeruginosa Strains PAO1 and PA14 J. Bacteriol., May 15, 2004; 186(10): 3270 - 3273. [Abstract] [Full Text] [PDF]

				C. Rodriguez, J. Tompkin, J. Hazel, and P. L. Foster Induction of a DNA Nickase in the Presence of Its Target Site Stimulates Adaptive Mutation in Escherichia coli J. Bacteriol., October 15, 2002; 184(20): 5599 - 5608. [Abstract] [Full Text] [PDF]

				E. S. Slechta, J. Liu, D. I. Andersson, and J. R. Roth Evidence That Selected Amplification of a Bacterial lac Frameshift Allele Stimulates Lac+ Reversion (Adaptive Mutation) With or Without General Hypermutability Genetics, July 1, 2002; 161(3): 945 - 956. [Abstract] [Full Text] [PDF]

				J. Zhao, H.-C. E. Leung, and M. E. Winkler The miaA Mutator Phenotype of Escherichia coli K-12 Requires Recombination Functions J. Bacteriol., March 1, 2001; 183(5): 1796 - 1800. [Abstract] [Full Text]

				H. J. Bull, G. J. McKenzie, P. J. Hastings, and S. M. Rosenberg Evidence That Stationary-Phase Hypermutation in the Escherichia coli Chromosome Is Promoted by Recombination Genetics, April 1, 2000; 154(4): 1427 - 1437. [Abstract] [Full Text]

				L. Ren, A. A. M. A. Mamun, and M. Z. Humayun Requirement for Homologous Recombination Functions for Expression of the mutA Mistranslator tRNA-Induced Mutator Phenotype in Escherichia coli J. Bacteriol., March 1, 2000; 182(5): 1427 - 1431. [Abstract] [Full Text]