Analysis of 2-amino-N6-hydroxyadenine-induced mutagenesis in phage M13mp2

Riboswitches as Drug Targets for Antibiotics

Riboswitches reside in the untranslated region of RNA and regulate genes involved in the biosynthesis of essential metabolites through binding of small molecules. Since their discovery at the beginning of this century, riboswitches have been regarded as potential antibacterial targets. Using fragment screening, high-throughput screening and rational ligand design guided by X-ray crystallography, lead compounds against various riboswitches have been identified. Here, we review the current status and suitability of the thiamine pyrophosphate (TPP), flavin mononucleotide (FMN), glmS, guanine, and other riboswitches as antibacterial targets and discuss them in a biological context. Further, we highlight challenges in riboswitch drug discovery and emphasis the need to develop riboswitch specific high-throughput screening methods.

Nucleoside and nucleobase analog mutagens

Compounds with structures close to those of normal nucleosides or nucleobases may be incorporated into cells and then become constituents of their DNA. Proliferation of such cells could yield mutants. In this article, the current status of studies on such nucleoside and nucleobase analogs is described. Base mispairing mechanisms for these analogs are discussed in light of recent biochemical and biophysical findings.

Mutagenicity of 5-bromouracil and N6-hydroxyadenine studied by yeast oligonucleotide transformation assay

The mutagenicity of 5-bromouracil (BrU) and N6-hydroxyadenine (HA) was tested by means of the yeast oligonucleotide transformation procedure. BrU-containing oligonucleotide was not mutagenic; although two mutants (per 200 micrograms oligonucleotide) were obtained, they were attributed to base insertion or base substitution at positions different from BrU. This result supports the view that BrU mutagenesis is dependent on intracellular nucleotide pool imbalance. In contrast, HA-containing oligonucleotide was highly mutagenic; 56 mutants (per 140 micrograms oligonucleotide) were obtained. Of 21 induced mutants examined, 20 had G and one had C at the HA position, a result indicating that HA-->G changes took place. To provide back-up evidence, we carried out a general reversion assay for base HA using a set of yeast tester strains, and the results showed that HA induces exclusively AT-to-GC and GC-to-AT transitions. We conclude that in S. cerevisiae HA is a classic base analog mutagen, causing AT-to-GC and GC-to-AT transitions by ambiguous base pairing. The present work has clearly demonstrated the usefulness of the oligonucleotide transformation procedure for elucidating mutagenicity of modified bases.

The synthesis and properties of oligodeoxyribonucleotides containing N6-methoxyadenine

Oligodeoxyribonucleotides containing N6-methoxyadenine (M) have been synthesized. The order of stability of duplexes consisting of synthesized oligodeoxyribonucleotides, 5'd(CCTGGTAXCAGGTCC)3'-5'd(GGACCTGNTACCAGG)3' (X = M, A, G. N = A, G, T, C), was M: A (Tm = 52 degrees C) greater than M: T (50 degrees C) greater than M: G (48 degrees C) greater than M: C (46 degrees C) observed by thermal denaturation in a buffer of 0.01 M Na cacodylate, and 0.1 M NaCl at pH 7.0. The Tms are within a range of 6 degrees of difference, which is smaller than those of Tms of the duplexes containing A:N pairs (11 degrees) and G:N pairs (11 degrees). DNA replication study on a template-primer system, 5'd(32p-CAGCTTTCGC)3' 3'd(GTCGAAAGCGMAGTCG)5', showed that TTP and dCTP were incorporated into DNA strands at a site opposite to M by Klenow DNA polymerase, but dATP and dGTP were not.

The genetic toxicology of 2-amino-N6-hydroxyadenine in eukaryotic organisms: significance for genetic risk assessment

A collaborative study was designed to assess the mutagenicity of 2-amino-N6-hydroxylaminopurine (AHA) in a wide variety of eukaryotic assays systems in terms of potency and specificity. Earlier studies in Salmonella and Neurospora had shown that AHA was an extremely potent mutagen which appeared to cause predominantly AT to GC base-pair transitions. This discovery was viewed as an unusual opportunity to explore the general utility of different eukaryotic assay systems for genetic risk assessment. The objective was to determine whether AHA would show comparable potency and specificity in those eukaryotic organisms used to evaluate mutagenic potential of environmental chemicals for the human population. The data presented in this report show that AHA was mutagenic in all the eukaryotic assays utilized; however, the level of effect was found to be assay system-dependent. In addition, in assays where other base analogs were used as positive controls, differences in relative potency were observed from those obtained in the earlier studies with Salmonella and Neurospora. When alkylating agents were used as positive controls in the higher eukaryotic assays, AHA was found to have a mutagenic potency comparable to ethylnitrosourea (ENU), ethyl methanesulfonate (EMS) or methyl methanesulfonate (MMS) for many of the assays. With regard to mutagenic specificity, AHA appears to induce gene/point mutations in eukaryotic organisms, resulting predominantly from base-pair substitutions, predominantly AT to GC base-pair transitions; however, there was some unexplained variation in the ratio of these base-pair transitions and other transitions and transversions as a function of assay system. In addition, studies on the induction of micronuclei have shown that AHA induces chromosomal damage at high concentrations and low levels of survival.

Mutagenic properties of 2-amino-N6-hydroxyadenine in Salmonella and in Chinese hamster lung cells in culture

The mutagenicity of the base analogue, 2-amino-N6-hydroxyadenine (AHA), was tested in Salmonella typhimurium TA100 and TA98 and in Chinese hamster lung (CHL) cells. AHA showed very potent mutagenicity in TA100 without S9 mix, inducing 25,000 revertants/micrograms. The mutagenicity increased about 2-fold upon addition of S9 mix containing 10 microliters S9. AHA was found to be one of the strongest mutagens for TA100. Addition of S9 mix containing 100 microliters S9 induced no significant increase of revertants with AHA at amounts up to 50 ng per plate. AHA was also mutagenic for the frameshift mutant, TA98, without S9 mix, the mutagenicity for TA98 being about 1/1000 of that for TA100. When the mutagenicity of AHA was tested in CHL cells, with diphtheria toxin resistance (DTr) as a selective marker in the absence of S9 mix with a 3-h treatment of cells, DTr mutants increased dose-dependently at concentrations of 2.5-15 micrograms/ml. When cells were incubated with AHA for 24 h, a 200-fold increase in the number of DTr mutants was observed; the mutagenicity was 500-fold higher than that of ethyl methanesulfonate. This marked increase of mutagenicity by prolonged incubation may indicate that AHA induces mutations mainly after incorporation into DNA. The addition of a small amount of S9 increased the mutagenicity obtained with a 3-h treatment 2-fold, but a larger amount of S9 decreased the mutagenicity as was found with S. typhimurium TA100.