Relationship of histidine sensitivity to DNA damage and stress induced responses in mutagen sensitive mutants of Neurospora crassa

Genetic dissection of PARylation in the filamentous fungus Neurospora crassa

PARylation is a posttranslational protein modification carried out by PAR polymerases (PARPs). These enzymes function as ADP-ribose transferases that add polymers of ADP-ribose (PAR) to target proteins. PARP proteins have critical functions impacting the aspects of normal human health, such as aging, as well as disease development, particularly cancer. Recently, the powerful antitumor PARP inhibitor Olaparib was shown to be effective in blocking the progression of BRCA1/2-associated tumors, prompting Bruce Alberts to call for an expansion of cancer research beyond utilization of cancer cell lines to include model organisms, such as bacteria, yeast, worms, flies, and mice. Although Dr. Alberts did not specifically mention the filamentous fungus Neurospora crassa, it is now known that Neurospora is the only genetically tractable model eukaryote with completely dispensable PARylation. PARylation in Neurospora can be entirely eliminated by disruption of a single predicted ORF, encoding a nuclear localized PARP protein termed Neurospora PARP ortholog (NPO). We, thus, present this initial genetic characterization of PARylation in N. crassa as evidence of the supreme advantage of using Neurospora as a tool for the genetic dissection of PARP and PARylation and emphasize the power of this system to advance unparalleled contributions to knowledge in this field.

qiRNA is a new type of small interfering RNA induced by DNA damage

RNA interference pathways use small RNAs to mediate gene silencing in eukaryotes. In addition to small interfering RNAs (siRNAs) and microRNAs, several types of endogenously produced small RNAs have important roles in gene regulation, germ cell maintenance and transposon silencing. The production of some of these RNAs requires the synthesis of aberrant RNAs (aRNAs) or pre-siRNAs, which are specifically recognized by RNA-dependent RNA polymerases to make double-stranded RNA. The mechanism for aRNA synthesis and recognition is largely unknown. Here we show that DNA damage induces the expression of the Argonaute protein QDE-2 and a new class of small RNAs in the filamentous fungus Neurospora crassa. This class of small RNAs, known as qiRNAs because of their interaction with QDE-2, are about 20-21 nucleotides long (several nucleotides shorter than Neurospora siRNAs), with a strong preference for uridine at the 5' end, and originate mostly from the ribosomal DNA locus. The production of qiRNAs requires the RNA-dependent RNA polymerase QDE-1, the Werner and Bloom RecQ DNA helicase homologue QDE-3 and dicers. qiRNA biogenesis also requires DNA-damage-induced aRNAs as precursors, a process that is dependent on both QDE-1 and QDE-3. Notably, our results suggest that QDE-1 is the DNA-dependent RNA polymerase that produces aRNAs. Furthermore, the Neurospora RNA interference mutants show increased sensitivity to DNA damage, suggesting a role for qiRNAs in the DNA-damage response by inhibiting protein translation.

Modulation of the H2O2-induced SOS response in Escherichia coli PQ300 by amino acids, metal chelators, antioxidants, and scavengers of reactive oxygen species

The SOS chromotest is a simple colorimetric genotoxicity assay that monitors DNA repair by measuring the induction of the gene sfiA in Escherichia coli K-12. E. coli PQ300, a diagnostic SOS tester strain for the detection of oxidative genotoxins, carries a mutation in a key gene for antioxidative defense, oxyR. This mutation renders PQ300 more sensitive to oxidative genotoxins, particularly to H2O2. We found that induction of the SOS response by H2O2 in E. coli PQ300 is dependent on the composition of the incubation medium; a substantially reduced response was obtained in minimal phosphate buffered saline (PBS) as opposed to complex Luria broth (LB) medium. Supplementation of PBS with histidine or cysteine stimulated H2O2-induced SOS induction to levels exceeding those found in LB medium. Low concentrations of glutathione (20-70 microM) also enhanced the H2O2-induced SOS response in E. coli PQ300, whereas higher concentrations (> 150 microM) were protective. Preincubation of tester cells with the chelators o-phenanthroline, 2,2-dipyridyl, and ethylenediaminetetraacetic acid (EDTA) protected cells from the effects of H2O2, although EDTA was only partially effective. Pretreatment of PQ300 with the antioxidant ascorbic acid or the hydroxyl radical scavenger dimethyl sulfoxide also diminished the SOS response, whereas mannitol and glucose were ineffective. The results show that the net effect of H2O2-induced DNA damage is influenced by the balance of oxidative and antioxidative factors and, furthermore, can be modulated by constituents of the extracellular milieu.

Inducible responses to DNA damaging or stress inducing agents in Neurospora crassa

Two-dimensional polyacrylamide gel electrophoresis has been used to analyze proteins from wild type and mutagen sensitive mutants of Neurospora crassa under constitutive conditions and after treatment with mutagens and other stress inducing agents. Several proteins have been detected that are either induced or show changes in electrophoretic mobility in response to UV irradiation, 4-NQO, X-ray, paraquat and heat shock. Ten proteins were found to respond to more than one of the stress inducing agents, demonstrating a complex pattern of polypeptide inductions. The significance of these findings and the possible effects of some of these proteins on the DNA repair process and stress management are discussed.

Deoxyribonucleoside triphosphate pools in Neurospora crassa: effects of histidine and hydroxyurea

An effective HPLC method for detecting deoxyribonucleoside triphosphates in hyphae from the fungus Neurospora crassa has been developed. In rapidly growing cells the nucleotide levels vary from 11.8 pmoles/micrograms DNA for dGTP to 24.2 pmoles/micrograms DNA for dTTP. These levels fall by approximately one half in stationary-phase cultures but the ratio of each pool to dGTP remains the same. The dNTP pools in conidia are at least 5-fold lower than in rapidly growing cells. The pool sizes are the same in static and shaking cultures. When the ribonucleotide reductase inhibitor, hydroxyurea (30 mM), is added to rapidly growing cultures, DNA synthesis is stopped and the dGTP pool is reduced by 39%, while the size of the other pools remains the same. In the presence of 11 mM histidine, DNA synthesis is also stopped and the size of the dGTP pool reduced by 46% while the deoxypyrimidine pools are somewhat increased. This suggests that the toxicity of excess histidine in Neurospora may be due to its ability to interact with the ribonucleotide reductase, inactivating the enzyme. Histidine may react with the free radical at the active site, as does hydroxyurea.