Kenneth R. Peterson

Possible horizontal transfer of Drosophila genes by the mite Proctolaelaps regalis.

There is strong inferential evidence for recent horizontal gene transfer of the P (mobile) element to Drosophila melanogaster from a species of the Drosophila willistoni group. One potential vector of this transfer is a semiparasitic mite, Proctolaelaps regalis DeLeon, whose morphology, behavior, and co-occurrence with Drosophila are consistent with the properties necessary for such a vector. Southern blot hybridization, polymerase chain reaction (PCR) amplification, and DNA sequencing showed that samples of P. regalis associated with a P strain of D. melanogaster carried P element sequences. Similarly, Drosophila ribosomal DNA sequences were identified in P. regalis samples that had been associated with Drosophila cultures. These results have potentially important evolutionary implications, not only for understanding the mechanisms by which genes may be transferred between reproductively isolated species, but also for improved detection of some host-parasite and predator-prey relationships.

Viability of Escherichia coli K-12 DNA adenine methylase (dam) mutants requires increased expression of specific genes in the SOS regulon

SummaryWe have examined the level of expression of the SOS regulon in cells lacking DNA adenine methylase activity (dam-). Mud (Ap, lac) fusions to several SOS operons (recA, lexA, uvrA, uvrB, uvrD, sulA, dinD and dinF) were found to express higher levels of β-galactosidase in dam- strains than in isogenic dam+ strains. The attempted construction of dam- strains that were also mutant in one of several SOS genes indicated that the viability of methylase-deficient strains correlates with the inactivation of the SOS repressor (LexA protein). Consistent with this, the wild-type functions of two LexA-repressed genes (recA and ruv) appear to be required for dam- strain viability.

Genetics of DNA repair in bacteria

Abstract In E. coli , DNA damage or interruption of DNA replication by a variety of treatments results in the SOS response. A coordinately regulated system of genes is depressed, leading to a variety of processes which enhance DNA repair. Through this mechanism the cell can recover from potentially lethal treatments.

UV‐INDUCIBLE SOS RESPONSE IN Escherichia coli

When Escherichia coli is exposed to ultraviolet light, the SOS DNA repair system is induced. The accompanying cellular response results, in part, from derepression of approximately 20 unlinked operons whose products aid the cell in recovering from DNA damage. These genes and their functions include: (1) the uvrABC complex and uvrD, which are involved in excision repair, (2) components of the RecF recombination pathway (recA, ruv and recN), ( 3 ) genes involved in mutagenesis such as recA and umuDC, and finally, (4) the cell division inhibitor, sulA, which postpones cell division until the repair of DNA damage is completed. Regulation of the SOS response is mediated through the recA and IexA gene products. When E. coli is exposed to UV light, an inducing signal is generated that alters RecA protein to an activated form. The activated form of RecA protein is capable of promoting cleavage of LexA protein, a repressor of the various genes that made up the SOS regulon. Since the cleaved LexA protein is no longer able to function as a repressor, the result is increased expression of the SOS genes, thus enhancing the ability of the cell to recover from damage. As the damage is repaired, the level of signal drops and RecA is no longer activated. LexA repressor then accumulates and the SOS genes are again repressed. This review will cover recent work on the SOS response, including new information on the role of the SOS response in excision repair, recombination repair, SOS mutagenesis and SOS induction. More comprehensive reviews on the SOS system have appeared elsewhere (Walker, 1984 and Ossanna et al., 1986).

Increased expression of the Escherichia coli umuDC operon restores SOS mutagenesis in lexA41 cells

SummaryThe lexA41 allele of Escherichia coli encodes a semidefective mutant repressor that is also resistant to RecA facilitated cleavage. Cells harboring the lexA41 allele were found previously to repress only a subset of operons in the SOS regulon. lexA41 cells cannot promote SOS mutagenesis, presumably because one or more operons required for mutagenesis are repressed by this mutant repressor. Using the lac regulatory system to increase the expression of the umuDC operon, we were able to restore mutagenesis in the lexA41 mutant. We conclude that the products of the umuDC operon appear to be uniquely limiting in this mutant.

Differential Expression of SOS Genes in an E. Coli Mutant Producing Unstable LexA Protein Enhances Excision Repair But Inhibits Mutagenesis

The lexA41 mutant of E. coli is a UV-resistant derivative of another mutant, lexA3, which produces a repressor that is not cleaved following inducing treatments. lexA41 carried an additional mutation which changed amino acid 132 in the LexA protein from Ala to Thr. The resultant protein was unstable and was degraded both before and after an inducing treatment. This instability was greater at 42 degrees than at 30 degrees. The protein was more stable in Lon- mutants at both temperatures. lac operon fusions to most of the genes in the SOS regulon were used to show that the various damage-inducible genes were derepressed to different extents. uvrA, B, and D were almost fully derepressed. Consistant with this finding, the rate of removal of T4 endonuclease V-sensitive sites was more rapid in the UV-irradiated lexA41 mutant than in normal cells, suggesting a more active excision repair system. We propose that the instability of the LexA41 protein reduces the intracellular concentration of repressor to a level that allows a high level of excision repair. The additional observation that SOS mutagenesis was only weakly induced in a lexA41 uvrA- mutant implies that the mutant protein partially represses one or more genes whose products promote SOS mutagenesis.