The Interaction of Acridines with Nucleic Acids

Binding site for ethidium cation in the major groove of B-form DNA

Molecular modeling of the system composed of ethidium cation initially intercalated through the major and minor grooves of the B-DNA form of 5'GCATGC3' into the central AT pairs, with the phenyl group oriented either toward or away from the helical axis, led upon energy minimization to intercalation complexes only when the phenyl group was initially oriented away from the helical axis. For the other two cases, energy minimization led to extrusion of the ethidium from the helix to form unanticipated outside-bound complexes stabilized by specific hydrogen bonds. It is suggested that outside-bound major groove complexes involving the GXXXG sequence may play important roles in the reaction mechanism of ethidium intercalation and in its action as a drug.

A structural model for sequence-specific proflavin-DNA interactions during in vitro frameshift mutagenesis

Molecular models describing intermediates that may lead to proflavin-induced 1 bp deletions during in vitro polymerization by E. coli DNA polymerase I Klenow fragment are proposed. The models provide structural explanations for the fact that the induced frameshifts always occur opposite template bases that are adjacent to 5' pyrimidines and are based on the underlying hypothesis that the deletions arise because the polymerase passes by a template base without copying it. Because the most frequent mutations are opposite Pu in the template sequence 5' Py Pu 3', a single-strand loop-out model was constructed for this sequence and proflavin was added, using structures found in crystalline oligonucleotides and their complexes with proflavin. The model seeks to rationalize the roles of the 5' pyrimidine and proflavin in facilitating the bypass. Four potential roles for proflavin in mutagenesis are described: 1) stacking on the looped-out base; 2) stacking on the base pair immediately preceding the site of mutation; 3) hydrogen bonding with the 5' pyrimidine; 4) hydrogen bonding with the phosphate backbone. These models point to the possibility that a number of proflavin-DNA interactions may be involved. In contrast, modeling does not suggest a role for classically intercalated proflavin in frameshift mutagenesis arising during in vitro DNA polymerization.

Effects of proflavin and photoactivated proflavin on the template function of single-stranded DNA

DNA context-specific effects of the association of proflavin, single-stranded DNA and DNA polymerase on DNA polymerization reactions were examined. Frameshift mutations induced by the presence of proflavin during in vitro DNA replication of a single-stranded DNA template by the Klenow fragment of Escherichia coli DNA polymerase I were sequenced. More than 80% of the frameshifts were one base-pair deletions opposite purine bases that were immediately 3' to pyrimidines. Purines (Pu) that were not adjacent to pyrimidines (Py) were not deletion sites. The remaining deletions were opposite template pyrimidines that were also immediately 3' to another pyrimidine. All pyrimidine site deletions occurred in the context 5' PyPyPu 3'. In additional experiments, the site-specific inhibition of processive DNA polymerization by proflavin was examined. A novel inhibition of polymerization was found opposite all pyrimidines in the template when proflavin-template complexes were exposed to ten seconds of white light. This inhibition of polymerization is reversible. Longer photoactivation led to an altered pattern of DNA sequence-specific inhibition that was not reversible. The role of DNA sequence-specific interactions of proflavin with DNA in proflavin mutagenesis is discussed.

Purinergic innervation of the guinea-pig urinary bladder

1 A number of criteria for considering adenosine 5'-triphosphate (ATP) as a neurotransmitter in the guinea-pig urinary bladder have been examined. In addition, the effect of tachyphylaxis to ATP on the response to non-adrenergic, non-cholinergic nerve stimulation has been re-examined.2 Quinacrine fluorescence histochemistry revealed a population of nerve fibres, ganglion cells, and nerve bundles in the bladder which were not seen in either the iris or vas deferens, where adrenergic and cholinergic nerves predominate. The distribution and morphology of the quinacrine-positive nerves in the bladder were different from those observed with catecholamine fluorescence and cholinesterase histochemistry, and were unaffected by chemical sympathectomy.3 Release of ATP from the bladder during stimulation of intramural excitatory nerves, in the presence of atropine and guanethidine increased to 3-12 times prestimulation levels. Tetrodotoxin abolished both the contractile response and the increase in ATP release resulting from intramural nerve stimulation. There was no increase in ATP release during contraction resulting from direct muscle stimulation following nerve paralysis with tetrodotoxin.4 Sympathectomy with 6-hydroxydopamine did not affect release of ATP in response to intramural nerve stimulation.5 Release of ATP was dependent on the concentration of calcium ion in the medium.6 Contractions in response to non-adrenergic, non-cholinergic intramural nerve stimulation were closely mimicked by ATP, but not by acetylcholine or histamine.7 Adenosine and dipyridamole reduced the contractions to both ATP and non-cholinergic nerve stimulation.8 2-2'-Pyridylisatogen was not a specific blocker of either ATP or intramural nerve stimulation in the guinea-pig bladder. 2-Substituted imidazolines initiated spontaneous activity making it impossible to assess any blocking action that they may have had.9 Prostaglandins (E(1), E(2) and F(2alpha)) gave weak, slow contractions and an increase in spontaneous activity. Both the response to ATP and non-adrenergic, non-cholinergic nerve stimulation were greatly potentiated in the presence of prostaglandins.10 In the presence of indomethacin the response to non-adrenergic, non-cholinergic nerve stimulation was virtually abolished following desensitization to ATP.