SUMMARY Pheromone responses are highly context-dependent. For example, the C. elegans pheromone ascaroside C9 (ascr#3) is repulsive to wild-type hermaphrodites, attractive to wild-type males, and usually neutral to “social” hermaphrodites with reduced activity of the npr-1 neuropeptide receptor gene. We show here that these distinct behavioral responses arise from overlapping push-pull circuits driven by two classes of pheromone-sensing neurons. The ADL sensory neurons detect C9, and in wild-type hermaphrodites, drive C9 repulsion through their chemical synapses. In npr-1 mutant hermaphrodites, C9 repulsion is reduced by the recruitment of a gap junction circuit that antagonizes ADL chemical synapses. In males, ADL sensory responses are diminished; in addition, a second pheromone-sensing neuron, ASK, antagonizes C9 repulsion. The additive effects of these antagonistic circuit elements generate attractive, repulsive or neutral pheromone responses. Neuronal modulation by circuit state and sex, and flexibility in synaptic output pathways, may permit small circuits to maximize their adaptive behavioral outputs.

Intraspecific chemical communication is mediated by signals called pheromones. C. elegans secretes a mixture of small molecules (collectively termed dauer pheromone) that regulates entry into the alternate dauer larval stage and also modulates adult behavior via as yet unknown receptors. Here, we identify two G protein-coupled receptors (GPCRs) that mediate dauer formation in response to a subset of dauer pheromone components. The SRBC-64 and SRBC-66 GPCRs are members of the large Caenorhabditis-specific SRBC subfamily, and are expressed in the ASK chemosensory neurons, which are required for pheromone-induced dauer formation. Expression of both, but not each receptor alone, confers pheromone-mediated effects on heterologous cells. Identification of dauer pheromone receptors will allow a better understanding of the signaling cascades that transduce the context-dependent effects of ecologically important chemical signals.

The accumulation of mutations causes cell lethality and can lead to carcinogenesis. An important class of mutations, which are associated with mutational hotspots in many organisms, are those that arise by nascent strand misalignment and template-switching at the site of short repetitive sequences in DNA. Mutagens that strongly and specifically affect this class, which is mechanistically distinct from other mutations that arise from polymerase errors or by DNA template damage, are unknown. Using Escherichia coli and assays for specific mutational events, this study defines such a mutagen, 3′-azidothymidine [zidovudine (AZT)], used widely in the treatment and prevention of HIV/AIDS. At sublethal doses, AZT has no significant effect on frame shifts and most basesubstitution mutations. AT-to-CG transversions and deletions at microhomologies were enhanced modestly by AZT. AZT strongly stimulated the "template-switch" class of mutations that arise in imperfect inverted repeat sequences by DNA-strand misalignments during replication, presumably through its action as a chain terminator during DNA replication. Chain-terminating 2′-3′-didehydro 3′-deoxythymidine [stavudine (D4T)] and 2′-3′-dideoxyinosine [didanosine (ddI)] likewise stimulated template-switch mutagenesis. These agents define a specific class of mutagen that promotes template-switching and acts by stalling replication rather than by direct nucleotide base damage.] is a frontline drug in the treatment of HIV/AIDS. Its therapeutic effects arise by its incorporation during HIV reverse transcription, resulting in chain termination. Azidothymidine is genotoxic, particularly to mitochondria, presumably because mitochondrial DNA polymerase gamma readily incorporates AZT during DNA synthesis (reviewed in Ref. 1). However, AZT also induces formation of micronuclei, sister chromatid exchange events, and various chromosomal aberrations, suggesting it may also be incorporated into nuclear DNA at some level (2). Using the bacterium Escherichia coli as a model, we have shown that AZT blocks DNA replication and causes formation of single-strand DNA gaps and double-strand breaks (3). E. coli cells appear to tolerate a certain level of AZT by the combined action of the DNA damage response, homologous recombination, and exonuclease excision. The chain-terminating azidothymidine monophosphate does not appear to be removed by intrinsic DNA polymerase proofreading; rather, the exogenous 3′ to 5′ dsDNA exonuclease, exonuclease III (an ortholog of human APE1), is likely the enzyme that removes the residue from DNA during sublethal exposure, because ExoIII − (xthA) mutants are highly sensitive to the drug and have an enhanced DNA damage response (3). A 75 ng/mL dose of AZT reduces viability of xthA mutants about 10,000-fold; because this dose has negligible effects on wild-type cells, many AZT-monophosphate lesions can be removed by ExoIII to sustain replication and proliferative capacity.In this study we investigate the mutagenicity of AZT, at chronic, sublethal doses ∼100-fold ...

Misalignment of a nascent strand and the use of an alternative template during DNA replication, a process termed “template-switching”, can give rise to frequent mutations and genetic rearrangements. Mutational hotspots are frequently found associated with imperfect inverted repeats (“quasipalindromes” or “QPs”) in many organisms, including bacteriophage, bacteria, yeast and mammals. Evidence suggests that QPs mutate by a replication template-switch whereby one copy of the inverted repeat templates synthesis of the other. To study quasipalindrome-associated mutagenesis (“QPM”) more systematically, we have engineered mutational reporters in the lacZ gene of Escherichia coli, that revert to Lac+ specifically by QPM. We and others have shown that QPM is more efficient during replication of the leading strand than it is on the lagging strand. We have previously shown that QPM is elevated and that the leading-strand bias is lost in mutants lacking the major 3′ ssDNA exonucleases, ExoI and ExoVII. This suggests that one or both of these exonucleases more efficiently abort template-switches on the lagging strand. Here, we show that ExoI is primarily responsible for this bias and that its ability to be recruited by single-strand DNA binding protein plays a critical role in QPM avoidance and strand bias. In addition to these stand-alone exonucleases, loss of the 3′ proofreading exonuclease activity of the replicative DNA polymerase III also greatly elevates QPM. This may be because template-switching is initiated by base misincorporation, leading to polymerase dissociation and subsequent nascent strand misalignment; alternatively or additionally, the proofreading exonuclease may scavenge displaced 3′ DNA that would otherwise be free to misalign.