Two Genetically Separable Phases of Growth Inhibition Induced by Blue Light in Arabidopsis Seedlings1

High fluence-rate blue light (BL) rapidly inhibits hypocotyl growth in Arabidopsis, as in other species, after a lag time of 30 s. This growth inhibition is always preceded by the activation of anion channels. The membrane depolarization that results from the activation of anion channels by BL was only 30% of the wild-type magnitude in hy4, a mutant lacking the HY4 BL receptor. High-resolution measurements of growth made with a computer-linked displacement transducer or digitized images revealed that BL caused a rapid inhibition of growth in wild-type and hy4 seedlings. This inhibition persisted in wild-type seedlings during more than 40 h of continuous BL. By contrast, hy4 escaped from the initial inhibition after approximately 1 h of BL and grew faster than wild type for approximately 30 h. Wild-type seedlings treated with 5-nitro-2-(3-phenylpropylamino)-benzoic acid, a potent blocker of the BL-activated anion channel, displayed rapid growth inhibition, but, similar to hy4, these seedlings escaped from inhibition after approximately 1 h of BL and phenocopied the mutant for at least 2.5 h. The effects of 5-nitro-2-(3-phenylpropylamino)-benzoic acid and the HY4 mutation were not additive. Taken together, the results indicate that BL acts through HY4 to activate anion channels at the plasma membrane, causing growth inhibition that begins after approximately 1 h. Neither HY4 nor anion channels appear to participate greatly in the initial phase of inhibition.

Dynamic contrast-enhanced magnetic resonance imaging reveals stress-induced angiogenesis in MCF7 human breast tumors.

The mechanism of contrast enhancement of tumors using magnetic resonance imaging was investigated in MCF7 human breast cancer implanted in nude mice. Dynamic contrast-enhanced images recorded at high spatial resolution were analyzed by an image analysis method based on a physiological model, which included the blood circulation, the tumor, the remaining tissues, and clearance via the kidneys. This analysis enabled us to map in rapidly enhancing regions within the tumor, the capillary permeability factor (capillary permeability times surface area per voxel volume) and the fraction of leakage space. Correlation of these maps with T2-weighted spin echo images, with histopathology, and with immunohistochemical staining of endothelial cells demonstrated the presence of dense permeable microcapillaries in the tumor periphery and in intratumoral regions that surrounded necrotic loci. The high leakage from the intratumoral permeable capillaries indicated an induction of a specific angiogenic process associated with stress conditions that cause necrosis. This induction was augmented in tumors responding to tamoxifen treatment. Determination of the distribution and extent of this stress-induced angiogenic activity by contrast-enhanced MRI might be of diagnostic and of prognostic value.

An immunoglobulin mutator that targets G.C base pairs.

Hypermutation can be defined as an enhancement of the spontaneous mutation rate which the organism uses in certain types of differentiated cells where a high mutation rate is advantageous. At the immunoglobulin loci this process increases the mutation rate > 10(5)-fold over the normal, spontaneous rate. Its proximate cause is called the immunoglobulin mutator system. The most important function of this system is to improve antibody affinity in an ongoing response; it is turned on and off during the differentiation of B lymphocytes. We have established an in vitro system to study hypermutation by transfecting a rearranged mu gene into a cell line in which an immunoglobulin mutator has been demonstrated. A construct containing the mu gene and the 3' kappa enhancer has all the cis-acting elements necessary for hypermutation of the endogenous gene segments encoding the variable region. The activity of the mutator does not seem to depend strongly on the position of the transfected gene in the genome. The mutator is not active in transformed cells of a later differentiation stage. It is also not active on a transfected lacZ gene. These results are consistent with the specificity of the mutator system being maintained and make it possible to delineate cis and trans mutator elements in vitro. Surprisingly, the mutator preferentially targets G-C base pairs. Two hypotheses are discussed: (i) the immunoglobulin mutator system in mammals consists of several mutators, of which the mutator described here is only one; or (ii) the primary specificity of the system is biased toward mutation of G-C base pairs, but this specificity is obscured by antigenic selection.

Structural variation of the pseudoautosomal region between and within inbred mouse strains.

The pseudoautosomal region (PAR) is a segment of shared homology between the sex chromosomes. Here we report additional probes for this region of the mouse genome. Genetic and fluorescence in situ hybridization analyses indicate that one probe, PAR-4, hybridizes to the pseudoautosomal telomere and a minor locus at the telomere of chromosome 9 and that a PCR assay based on the PAR-4 sequence amplifies only the pseudoautosomal locus (DXYHgu1). The region detected by PAR-4 is structurally unstable; it shows polymorphism both between mouse strains and between animals of the same inbred strain, which implies an unusually high mutation rate. Variation occurs in the region adjacent to a (TTAGGG)n array. Two pseudoautosomal probes can also hybridize to the distal telomeres of chromosomes 9 and 13, and all three telomeres contain DXYMov15. The similarity between these telomeres may reflect ancestral telomere-telomere exchange.