Transposon-induced mutations in two loci of Listeria monocytogenes serotype 1/2a result in phage resistance and lack of N-acetylglucosamine in the teichoic acid of the cell wall.

Teichoic acid-associated N-acetylglucosamine and rhamnose have been shown to serve as phage receptors in Listeria monocytogenes serotype 1/2a. We generated and characterized two single-copy Tn916DeltaE mutants which were resistant to phage A118 and several other serotype 1/2a-specific phages. In one mutant the insertion was immediately upstream of the recently identified ptsHI locus, which encodes two proteins of the phosphoenolpyruvate-dependent carbohydrate uptake system, whereas in the other the insertion was immediately upstream of an operon whose most distal gene was clpC, involved in stress responses and virulence. Transduction experiments confirmed the association of the phage-resistant phenotype of these mutants with the transposon insertion. Phage A118 resistance of the mutants could be attributed to inability of the phage to adsorb onto the mutant cells, and biochemical analysis of cell wall composition showed that the teichoic acids of both mutants were deficient in N-acetylglucosamine. Rhamnose and other teichoic acid and cell wall components were not affected.     

The Social Environment of an Aggregating, Ant-Attended Treehopper (Hemiptera: Membracidae: Vanduzea arquata)

Many membracid treehoppers are attended by honeydew-harvesting ants. Ant mutualism often favors group living, which will in turn influence social interactios and communication. I investigated aspects of life history that underlie the social behavior of an aggregating, ant-attended treehopper. The number of adults, and their patterns of distribution, changes dramatically over the course of a season. Despite the relatively low vagility and high persistence in the same clump of host plants, individuals encounter a wide range of social environments. This aggregating species differs from solitary species in the clumped distribution of females, and possibly in the intensity of acoustic competition among males, but both aggregating and solitary species exhibit large temporal changes in density. A high degree of temporal and spatial variation in the social environment is probably characteristic of many insects and may be an important source of selection on insect communication.     

Quantitative Modeling of GRK-Mediated β2AR Regulation

We developed a unified model of the GRK-mediated β2 adrenergic receptor (β2AR) regulation that simultaneously accounts for six different biochemical measurements of the system obtained over a wide range of agonist concentrations. Using a single deterministic model we accounted for (1) GRK phosphorylation in response to various full and partial agonists; (2) dephosphorylation of the GRK site on the β2AR; (3) β2AR internalization; (4) recycling of the β2AR post isoproterenol treatment; (5) β2AR desensitization; and (6) β2AR resensitization. Simulations of our model show that plasma membrane dephosphorylation and recycling of the phosphorylated receptor are necessary to adequately account for the measured dephosphorylation kinetics. We further used the model to predict the consequences of (1) modifying rates such as GRK phosphorylation of the receptor, arrestin binding and dissociation from the receptor, and receptor dephosphorylation that should reflect effects of knockdowns and overexpressions of these components; and (2) varying concentration and frequency of agonist stimulation “seen” by the β2AR to better mimic hormonal, neurophysiological and pharmacological stimulations of the β2AR. Exploring the consequences of rapid pulsatile agonist stimulation, we found that although resensitization was rapid, the β2AR system retained the memory of the previous stimuli and desensitized faster and much more strongly in response to subsequent stimuli. The latent memory that we predict is due to slower membrane dephosphorylation, which allows for progressive accumulation of phosphorylated receptor on the surface. This primes the receptor for faster arrestin binding on subsequent agonist activation leading to a greater extent of desensitization. In summary, the model is unique in accounting for the behavior of the β2AR system across multiple types of biochemical measurements using a single set of experimentally constrained parameters. It also provides insight into how the signaling machinery can retain memory of prior stimulation long after near complete resensitization has been achieved.