Prohibitin, an evolutionarily conserved intracellular protein that blocks DNA synthesis in normal fibroblasts and HeLa cells.

Genes that act inside the cell to negatively regulate proliferation are of great interest because of their implications for such processes as development and cancer, but these genes have been difficult to clone. This report details the cloning and analysis of cDNA for prohibitin, a novel mammalian antiproliferative protein. Microinjection of synthetic prohibitin mRNA blocks entry into S phase in both normal fibroblasts and HeLa cells. Microinjection of an antisense oligonucleotide stimulates entry into S phase. By sequence comparison, the prohibitin gene appears to be the mammalian analog of Cc, a Drosophila gene that is vital for normal development.     

Rhizobium meliloti exopolysaccharides: genetic analyses and symbiotic importance.

Genetic experiments have indicated that succinoglycan (EPS I), the acidic Calcofluor-binding exopolysaccharide, of the nitrogen-fixing bacterium Rhizobium meliloti strain Rm1021 is required for nodule invasion and possibly for later events in nodule development on alfalfa and other hosts. Fourteen exo loci on the second megaplasmid have been identified that are required for, or affect, the synthesis of EPS I. Mutations in certain of these loci completely abolish the production of EPS I and result in mutants that form empty Fix- nodules. We have identified two loci, exoR and exoS, that are involved in the regulation of EPS I synthesis in the free-living state. Certain exo mutations which completely abolish EPS I production are lethal in an exoR95 or exoS96 background. Histochemical analyses of the expression of exo genes during nodulation using exo::TnphoA fusions have indicated that the exo genes are expressed most strongly in the invasion zone. In addition, we have discovered that R. meliloti has a latent capacity to synthesize a second exopolysaccharide (EPS II) that can substitute for the role(s) of EPS I in nodulation of alfalfa but not of other hosts. Possible roles for exopolysaccharides in symbiosis are discussed.     

A study of the application of Kohonen-type neural networks to the Travelling Salesman Problem

It is observed that animals often have to resolve difficult tasks of optimization and that this process can be studied by applying the formal framework of neural networks to a simple problem such as the Travelling Salesman Problem. Existing work is reviewed with particular emphasis on recent studies using self-organizing networks. An algorithm is described in which general principles developed by Kohonen are applied to the Travelling Salesman Problem. Simulation results are given for problem sets of up to 10,000 cities. The routes generated are reported as being slightly longer than those produced by simulating annealing; compute time is lower and scales less than quadratically with problem size. It is suggested that the ability to perform optimization is an emergent computational property not just of the Kohonen model but of any mechanism capable of producing topology-preserving mappings, including mechanisms within the brain.     

Genetic counselor training: a review and considerations for the future.

The first training program for genetic counselors began in 1969. Since then a number of other programs have been developed and more than 650 individuals have graduated from these programs. This article reviews the development and current status of training opportunities for genetic counselors. Twelve programs that currently grant a master's-level degree in genetic counseling are reviewed. Other areas, such as certification and licensure, that reflect genetic counseling training or such issues of professional growth as continuing education and career advances are addressed.     

Separation of large DNA restriction fragments on a size-exclusion column by a nonideal mechanism

Double-strand DNA (dsDNA) restriction fragments were chromatographed on the DuPont Bioseries GF-250 column. Two anomolous chromatographic properties were observed. (1) A triphasic dependence of retention on dsDNA chain length was observed. Small DNA fragments (less than 500 base pairs) displayed typical size exclusion, intermediate size DNA (800-5000 base pairs) eluted in the void volume, and larger DNA fragments were increasingly retained. (2) The void volume for nucleic acids was less than that for large polypeptides. The retention of moderately large DNA fragments increased linearly as the square root of the chain length over the range 5.5 to 50 kilobase pairs (ca. 3-30 X 10(6) Mr). A number of eluant manipulations were carried out in order to examine the mechanism by which the larger DNA fragments were being retained and separated. Evidence was not obtained to support either ion exchange or reverse phase as the retention mechanism. The usefulness of such a column for molecular biological manipulations is illustrated by the rapid isolation of homogeneous viral DNA fragments resected from their cloning vectors with restriction endonucleases.     

All nod genes of Rhizobium meliloti are involved in alfalfa nodulation by exo mutants.

Nodulation of alfalfa by exoB mutants of Rhizobium meliloti occurred without root hair curling or infection thread formation. nod exoB double mutants had the same nodulation deficiency as single nod mutants. Therefore, all the known nod genes are involved in nodule induction by exoB mutants, which apparently occurs via intercellular invasion.     

Amikacin disrupts the cell envelope of Pseudomonas aeruginosa ATCC 9027

Amikacin, an aminoglycoside known to inhibit protein synthesis, was found to perturb the outer membrane of a sensitive Pseudomonas aeruginosa strain (ATCC 9027). This perturbation was monitored using electron microscopy and biochemical analyses. Following exposure to 20 micrograms amikacin/mL for 15 min, the outer membrane of exponentially growing cells lost 15% of its protein, 18% of its lipopolysaccharide, and 18% of its phosphate. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis showed that the whole spectrum of outer membrane protein and lipopolysaccharide was affected. Similarly, atomic absorption spectrophotometry revealed that magnesium and calcium were also lost. When cells were treated with amikacin, electron microscopy of negative stains showed a substantial increase in outer membrane blebbing. Freeze fractures revealed changes in membrane fracture pattern and particle distribution, and thin sections revealed a sequential disruption of the cell envelope beginning at the outer membrane and ending at the plasma membrane. This study supports the proposal that aminoglycoside antibiotics cross the outer membrane of Pseudomonas aeruginosa by displacing metal cations necessary to stabilize the organic constituents of the membrane. Their removal results in loss of the outer membrane and the formation of transient small holes which permit the antibiotic access to the cytoplasmic membrane where it is transported into the cytoplasm.     

Selective association and transport of Campylobacter jejuni through M cells of rabbit Peyer's patches

M cells in the Peyer's patches may facilitate transport of pathogens such as Campylobacter jejuni from the intestine. We evaluated this hypothesis by using electron microscopy to examine Peyer's patches in ligated adult rabbit ileal loops inoculated with 5-mL suspensions of 10(9) cfu/mL of Campylobacter jejuni. Peyer's patches taken at intervals from 15 min to 2 h after inoculation of loops in anaesthetized rabbits provided evidence that Campylobacter jejuni selectively adhered to M cells as opposed to absorptive epithelial cells and was transported, apparently intact, into the M cell follicle. Although intercellular organisms were seen within the follicle, many others were phagocytosed by lymphoid cells. The proximity of the lymphatic and blood circulatory systems to the M cell follicle makes this a probable route for systemic spread of Campylobacter jejuni.