Dimethyl sulfoxide at 2.5% (v/v) alters the structural cooperativity and unfolding mechanism of dimeric bacterial NAD+ synthetase

Dimethyl sulfoxide (DMSO) is commonly used as a cosolvent to improve the aqueous solubility of small organic compounds. Its use in a screen to identify novel inhibitors of the enzyme NAD(+) synthetase led to this investigation of its potential effects on the structure and stability of this 60-kD homodimeric enzyme. Although no effects are observed on the enzyme's catalytic activity, as low as 2.5% (v/v) DMSO led to demonstrable changes in the stability of the dimer and its unfolding mechanism. In the absence of DMSO, the dimer behaves hydrodynamically as a single ideal species, as determined by equilibrium analytical ultracentrifugation, and thermally unfolds according to a two-state dissociative mechanism, based on analysis by differential scanning calorimetry (DSC). In the presence of 2.5% (v/v) DMSO, an equilibrium between the dimer and monomer is now detectable with a measured dimer association constant, K(a), equal to 5.6 x 10(6)/M. DSC curve analysis is consistent with this finding. The data are best fit to a three-state sequential unfolding mechanism, most likely representing folded dimer <==> folded monomer <==> unfolded monomer. The unusually large change in the relative stabilities of dimer and monomer, e.g., the association equilibrium shifts from an infinitely large K(a) down to approximately 10(6)/M, in the presence of relatively low cosolvent concentration is surprising in view of the significant buried surface area at the dimer interface, roughly 20% of the surface area of each monomer is buried. A hypothetical structural mechanism to explain this effect is presented.     

Female choice in Drosophila: evidence from Hawaii and implications for evolutionary biology

Details of female choice of mate in Drosophila silvestris of Hawaii strikingly parallels epigamic behavioral systems in many other animals and may be common in other species of Drosophilidae. Females respond selectively to male circling, wing displays, songs and tactile stimulation with foreleg cilia, a quantitative character that is highly variable in some populations. I hypothesize that the female can exert choice based on these cues from individual males that differ genetically by quantitative trait loci. Laboratory tests show that one third of courting males are repeatedly rejected in favor of a minority of alpha males. This result imposes non-random mating at the local population level. Past multiple-choice lab tests, widely used to measure isolation between pairs of populations or species of Drosophila may be flawed, since random mating has been assumed in the interpretation of results. Pre-mating sexual selection is clearly a powerful intrapopulation force in population biology. This view creates difficulties for discerning any proposed simultaneous interpopulation selective events in the presence of strong female choice. The long-held theory assuming that there is significant selection for pre-mating isolation between groups is questionable.     

A Spiking Neuron Model for Binocular Rivalry

We present a biologically plausible model of binocular rivalry consisting of a network of Hodgkin-Huxley type neurons. Our model accounts for the experimentally and psychophysically observed phenomena: (1) it reproduces the distribution of dominance durations seen in both humans and primates, (2) it exhibits a lack of correlation between lengths of successive dominance durations, (3) variation of stimulus strength to one eye influences only the mean dominance duration of the contralateral eye, not the mean dominance duration of the ipsilateral eye, (4) increasing both stimuli strengths in parallel decreases the mean dominance durations. We have also derived a reduced population rate model from our spiking model from which explicit expressions for the dependence of the dominance durations on input strengths are analytically calculated. We also use this reduced model to derive an expression for the distribution of dominance durations seen within an individual.     

Urological and medical evaluation of men with erectile dysfunction

As the medical understanding of erectile dysfunction has evolved, the approach to its evaluation must also change. A good doctor/patient rapport is crucial to making patients more comfortable talking about their sexual function. Questionnaires help elicit specific information on which to base the diagnosis and find the etiology, along with examination of the genitals and hormone and other assays. If oral therapy is unsuccessful, other studies may find underlying conditions, leading to appropriate treatment.     

The role of TFIIB-RNA polymerase II interaction in start site selection in yeast cells

Previous studies have established a critical role of both TFIIB and RNA polymerase II (RNAPII) in start site selection in the yeast Saccharomyces cerevisiae. However, it remains unclear how the TFIIB–RNAPII interaction impacts on this process since such an interaction can potentially influence both preinitiation complex (PIC) stability and conformation. In this study, we further investigate the role of TFIIB in start site selection by characterizing our newly generated TFIIB mutants, two of which exhibit a novel upstream shift of start sites in vivo. We took advantage of an artificial recruitment system in which an RNAPII holoenzyme component is covalently linked to a DNA-binding domain for more direct and stable recruitment. We show that TFIIB mutations can exert their effects on start site selection in such an artificial recruitment system even though it has a relaxed requirement for TFIIB. We further show that these TFIIB mutants have normal affinity for RNAPII and do not alter the promoter melting/scanning step. Finally, we show that overexpressing the genetically isolated TFIIB mutant E62K, which has a reduced affinity for RNAPII, can correct its start site selection defect. We discuss a model in which the TFIIB–RNAPII interaction controls the start site selection process by influencing the conformation of PIC prior to or during PIC assembly, as opposed to PIC stability.