A ruthenium dimer complex with a flexible linker slowly threads between DNA bases in two distinct steps

Several multi-component DNA intercalating small molecules have been designed around ruthenium-based intercalating monomers to optimize DNA binding properties for therapeutic use. Here we probe the DNA binding ligand [μ-C4(cpdppz)₂(phen)₄Ru₂]⁴⁺, which consists of two Ru(phen)₂dppz²⁺ moieties joined by a flexible linker. To quantify ligand binding, double-stranded DNA is stretched with optical tweezers and exposed to ligand under constant applied force. In contrast to other bis-intercalators, we find that ligand association is described by a two-step process, which consists of fast bimolecular intercalation of the first dppz moiety followed by ∼10-fold slower intercalation of the second dppz moiety. The second step is rate-limited by the requirement for a DNA-ligand conformational change that allows the flexible linker to pass through the DNA duplex. Based on our measured force-dependent binding rates and ligand-induced DNA elongation measurements, we are able to map out the energy landscape and structural dynamics for both ligand binding steps. In addition, we find that at zero force the overall binding process involves fast association (∼10 s), slow dissociation (∼300 s), and very high affinity (K_d ∼10 nM). The methodology developed in this work will be useful for studying the mechanism of DNA binding by other multi-step intercalating ligands and proteins.     

Host-parasite relationship 1- alkaline and acid phosphatase distribution in the silk glands of parasitized larvae of the rice mothCorcyra cephalonica [Lep.: Pyralidae]

The alkaline and acid phosphatases were demonstrated histochemically along the whole length of the silk gland in normal and paralysed larvae of the rice mothCorcyra cephalonica. The activity of both enzymes was found to be much reduced as a result of the toxin injected in the host's body by the parasiteBracon hebetor. In both the normal and paralysed larvae the two enzymes were found to be more intense in the nuclei than in the cytoplasm. Both enzymes could not be detected in the cells of the productive tube region of the glands in paralyzed larvae.     

Host-parasite relationship 2- cholinesterase activity of the larvae of the rice mothCorcyra cephalonica [Lep.: Pyralidae] parasitized byBracon hebetor [Hym.: Braconidae]

The parasitic waspBracon hebetor Say paralyses its host, the larvae of the rice mothCorcyra cephalonica Staint, before oviposition. The effect of injected venom on the activity of cholinesterase was studied. The ChE activity of the paralysed larvae showed a partial inhibition of about 40% than that of the normal larvae. Such inhibition in ChE activity interferes with the synaptic transmission and/or may result in neuromuscular block causing the observed paralysis.