Cell death caused by arrested replication of damaged or structurally altered DNA can be avoided in prokaryotic and eukaryotic cells by multiple DNA polymerases that are specialized to bypass DNA damage. Some of these polymerases perform such translesion DNA synthesis of specific types of damage with high genetic fidelity. However, they exhibit greatly reduced fidelity when they operate on undamaged DNA or on DNA with lesions that are (apparently) not cognate substrates. The low fidelity of some of these specialized polymerases when copying undamaged DNA may be physiologically functional, including generating immunoglobulin diversity.
The polychlorinated biphenyl (PCB) residues in the aquatic sediments from six PCB spill sites showed changes in PCB isomer and homolog (congener) distribution that indicated the occurrence of reductive dechlorination. The PCB dechlorinations exhibited several distinct congener selection patterns that indicated mediation by several different localized populations of anaerobic microorganisms. The higher (more heavily chlorinated) PCB congeners that were preferentially attacked by the observed dechlorination processes included all those that are either pharmacologically active or persistent in higher animals. All the lower (less heavily chlorinated) PCB congeners formed by the dechlorinations were species that are known to be oxidatively biodegradable by the bacteria of aerobic environments.
Heteroduplexes with mismatches at four sites were constructed from separated strands of X DNA and used to transfect Escherichia coli under recombinationless conditions. The output phages from 967 single cells in one experiment and 1016 in another were analyzed to determine the pattern of mismatch repair. A wide range of repair frequencies was found among the mismatches studied. Repair involving two or more close sites in the same heteroduplex occurs much more often on the same strand than on opposite strands. Analysis of the pattern of repair suggests that repair tracts initiate at mismatches, propagate preferentially in the5'-'3' direction, and extend an average distance of ca 3000 nucleotides.The repair of mismatched bases in DNA is thought to contribute to gene conversion and to be involved in certain other phenomena in genetic recombination (1-4). Mismatched bases in the heteroduplex regions formed during genetic recombination are postulated to provoke localized excision on one polynucleotide strand followed by repair synthesis using the intact strand as template. Although mismatch repair has not yet been demonstrated in vitro, evidence for its occurrence has been obtained from genetic studies of bacterial transformation and transfection with artificially constructed heteroduplex DNA (5-11).Wildenberg and Meselson (10) found that when Escherichia coli is transfected with heteroduplex molecules of bacteriophage X, different amber/wild-type mismatches are repaired to homozygous wild type at rates characteristic of the individual amber. Well-separated pairs of mismatches were repaired independently; however, there was an indication that sites less than about 2000 nucleotides apart are sometimes repaired in a single event.The present experiments further characterize the process of mismatch repair. Separated DNA strands from two genetically marked strains of X were annealed to form the two possible heteroduplexes. E. coli was transfected with each heteroduplex and the genotypes of the output phages from individual cells were determined. A wide range of repair frequencies is found among the six markers studied. Even for a given mismatch in a particular heteroduplex, the frequency of repair on one strand can differ considerably from that on the other. Repair involving two or more close sites in the same heteroduplex occurs much more often on the same strand than on opposite strands. The pattern of repair suggests that repair events initiate at mismatches and propagate preferentially in the 5`-g'3' direction, extending an average distance of ca 3000 nucleotides. MATERIALS AND METHODSBacteria and phage strains are listed in Table 1. Phage stocks were prepared on C600 and C600.3 as previously described (10). All phages used for DNA preparations have the deletions b2 and bio69 and are therefore att-int-red-. DNA strands were prepared and annealed as described earlier (22) except that the D20-H20 gradient step was omitted. Transfection with each single strand preparation after self-annealing showed 4135 complem...
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