Pairwise Comparisons Using Trimmed Means or M-Estimators when Working with Dependent Groups

For J dependent groups, let θ_j, j = 1, …, J, be some measure of location associated with the jth group. A common goal is computing confidence intervals for the pairwise differences, θ_j — θ_k, j < k, such that the simultaneous probability coverage is 1 — α. If means are used, it is well known that slight departures from normality (as measured by the Kolmogorov distance) toward a heavy-tailed distribution can substantially inflate the standard error of the sample mean, which in turn can result in relatively low power. Also, when distributions differ in shape, or when sampling from skewed distributions with relatively light tails, practical problems arise when the goal is to obtain confidence intervals with simultaneous probability coverage reasonably close to the nominal level. Extant theoretical and simulation results suggest replacing means with trimmed means. The Tukey-McLaughlin method is easily adapted to the problem at hand via the Bonferroni inequality, but this paper illustrates that practical concerns remain. Here, the main result is that the percentile t bootstrap method, used in conjunction with trimmed means, gives improved probability coverage and substantially better power. A method based on a one-step M-estimator is also considered but found to be less satisfactory.     

Regulation of L-arabinose transport in Salmonella typhimurium LT2

Summary The inducible L-arabinose transport system was characterized in Salmonella typhimurium LT2. Only one L-arabinose transport system with a Km of 2x10^-4 M was identified. The results suggested that araE may be the only gene which codes for L-arabinose transport activity under the conditions tested. An araE-lac fusion strain was used to study the induction of the araE gene. No araE expression was detected when the L-arabinose concentration was lower than 1 mM. The expression of araE reached a maximum in the presence of 50 mM L-arabinose, and was significantly reduced in the presence of D-glucose. Expression of the araBAD and araE genes was coordinately regulated. The concentration of L-arabinose that allowed maximum araBAD gene expression was 50-fold lower in an araE ⁺ strain compared to an araE strain.     

Protein products of the bithorax complex in Drosophila

A sequence from the Ubx 5' exon in the bithorax complex of Drosophila melanogaster was expressed as a fusion protein in bacteria. This protein was used to raise rabbit antisera and monoclonal antibodies. These antibodies detect antigens that, on protein blots and by immunofluorescence on whole mounts of imaginal discs, show the predicted segmental distribution of Ubx products. These products are predominantly, if not totally, localized in the cell nucleus. In the embryonic nervous system nuclei are labeled from the second thoracic segment to the eighth abdominal segment. There is no labeling in homozygous Df bxd100 embryos.     

The shape of L-arabinose isomerase from Escherichia coli.

L-Arabinose isomerase, EC 5.3.1.4, catalyzes the conversion of L-arabinose to L-ribulose, the first step in the catabolism of L-arabinose by Escherichia coli B/r. Patrick and Lee (1969) J. Biol. Chem. 244, 4277--4283) demonstrated that native L-arabinose isomerase is composed of six identical subunits of approximately Mr = 60,000. In this paper we describe an electron microscopy study of the arrangement of the six identical subunits. The isomerase is seen in two distinctly different orientations. The first has three subunits visible, with a 3-fold axis of symmetry, corresponding to a face-on view of two stacked, eclipsed trimers. The second orientation is rectangular in shape with 2-fold symmetry; suggesting a side-on view of the stacked trimers. The six identical subunits are thus arranged with D3 symmetry as in a trigonal prism. Measurements were made on the maximum profile of the three 2-fold axes of symmetry of the face-on orientations, and of both the long and short dimensions of the side-on orientation. The best estimate for the maximum profile of the 2-fold axes of symmetry of the face-on view is 106 +/- 8 A, using glutamine synthetase as an internal size standard. Measurements from micrographs of the isomerase alone, using an external magnification calibration, give the following results: for the maximum profile of the three 2-fold axes of symmetry of the face-on view, 132 +/- 7 A; for the long axis of the side-on view, 136 +/- 10 A; and for the short axis, 105 +/- 6 A. These measurements are consisting with the interpretation of the profiles as representing two different orientations of the L-arabinose isomerase.