Microbiology: Gut response to toxin

Science 313, 848–851 (2006)

Certain strains of the bacterium Escherichia coli manufacture a toxin that introduces double-strand breaks into mammalian DNA, according to new research. The damaged cells swell up and stop dividing.

Eric Oswald of the National Veterinary College of Toulouse, France, and his colleagues identified a gene cluster responsible for producing the toxin in different strains of E. coli. The cluster was found both in strains that cause infection and in strains that live in the human gut, including one used for intestinal 'probiotic' treatment.

The bugs may benefit from blocking intestinal cell division, as this could help them to colonize the gut. The DNA damage caused by the bacteria might predispose the host organism to cancer.

Human genetics: Deletion diagnosis

Nature Genet. doi:10.1038/ng1853; 10.1038/ng1858 and 10.1038/ng1862 (2006)

A new clinical syndrome has been identified in individuals with learning difficulties. The syndrome, as yet unnamed, is caused by a very small deletion in chromosome 17. It is expected to explain about 1% of the undiagnosed cases of mental retardation.

The deletion was identified by three independent research teams using genomic tools. Bigger chromosomal abnormalities that cause mental retardation, such as Down's syndrome, can be seen under the microscope.

The deleted region is flanked by DNA sequences that are typical of other areas prone to deletion. It carries only about half a dozen genes. One of these is the so-called tau gene, MAPT, which is mutated in some neurodegenerative disorders, including Alzheimer's disease.

Astronomy: Line of sight

Astrophys. J. preprint available at http://www.arxiv.org/abs/astro-ph/0606279 (2006)

Astronomers have used the circling dance of two stars to determine the distance to one of the Milky Way's neighbours, galaxy M33. The measurement pushes to its limit one of the few direct techniques for mapping space.

Alceste Bonanos of the Carnegie Institution of Washington and her colleagues studied how the brightness of the stellar pair changed as one star eclipsed the other. From the orbit's properties, the team estimated the two stars' masses, and thus their absolute brightness.

Then, by comparing the observed brightness to that predicted, they calculated the stars' host galaxy to be about 3.1 million light years away.

The result is at the upper end of the range obtained by other methods. It is also the greatest distance over which the binary technique has been applied: the stars would be unresolvable if any farther away.

Cancer Biology: Dog cancer has wolf roots

Credit: AFLO/NATUREPL.COM

Cell doi:10.1016/j.cell.2006.05.051 (2006)

An unusual, contagious cancer that affects dogs probably emerged in wolves or their relatives between 200 and 2,500 years ago, according to researchers in the United Kingdom and the United States.

Canine transmissible venereal tumour (CTVT) is passed from pooch to pooch by transmission of the tumour cells themselves, usually when the animals have sex. Robin Weiss of University College London, UK, and his colleagues compared the genotype of the tumour to that of more than 400 dogs in 85 breeds. The tumour was most closely related to dogs such as wolves and huskies (pictured above). The researchers estimated the age of the cancerous cell line by counting mutations.

Immunology: Weakened zinc link

Nature Immunol. doi:10.1038/ni1373 (2006)

Zinc deficiency is known to impair the immune system, but the mechanism for this has been unclear. New work hints that zinc may have a signalling role in the development of some immune cells.

Toshio Hirano of the RIKEN Research Center for Allergy and Immunology in Kanagawa, Japan, and his colleagues studied dendritic cells, which prime other immune cells to respond to a pathogen's proteins. They showed that the zinc concentration in dendritic cells fell when they were stimulated in a way that mimics infection. Moreover, artificially reducing the zinc concentration in dendritic cells stimulated them to mature as they do when faced with a pathogen. The researchers suggest that zinc deficiency may alter immune function in part by knocking this system off balance.

Animal behaviour: Fish farmers

Biol. Lett. doi:10.1098/rsbl.2006.0528 (2006)

Damselfish, residents of coral reefs, are known to be conscientious farmers, cultivating crops of algae for their own consumption. A detailed study of the species' habitats has now revealed that some algae are equally dependent on these fish.

Hiroki Hata and Makoto Kato of Kyoto University, Japan, collected hundreds of samples of algae from both within and outside damselfish territories. One Polysiphonia algal species dominated the farms of the damselfish Stegastes nigricans and was found nowhere else.

This suggests that the two species rely on each other: a situation described as obligate cultivation mutualism. Fungi-cultivating insects and crop-growing humans have a similar set-up.

Chemical biology: Improving nature

J. Am. Chem. Soc. doi:10.1021/ja061099y (2006)

As catalysts, enzymes are hard to improve on. But Ryan Mehl and his colleagues at the Franklin and Marshall College in Lancaster, Pennsylvania, have succeeded. By inserting a non-natural amino acid into the enzyme nitroreductase, they increased its activity.

Nitroreductase activates the anticancer prodrug CB1954, converting it to a toxic form. The researchers measured the effect of replacing a key amino acid in the enzyme's active site with one of eight different non-natural amino acids, each of which was predicted to improve binding of the prodrug. One of these made the enzyme 30 times more efficient than natural nitroreductase.

Cell biology: Mutated in the middle

Credit: W. HO & M. FITZGERALD/ASBMB

J. Biol. Chem. 281, 20494–20502 (2006)

The so-called 'guardian of the genome', p53, protects against many cancers — unless mutations prevent it binding to DNA. Crystal structures of the p53–DNA complex show how this can happen.

Crystal structures of a single p53 protein bound to DNA have previously been used to explain the effect of some mutations implicated in cancer. But p53 actually binds in fours, as pairs of dimers. Ronen Marmorstein and his colleagues from the University of Pennsylvania in Philadelphia studied the structure of a p53 dimer bound to DNA (pictured above; DNA shown in red and grey).

They found that many faulty p53 proteins have mutations in the area where the two units of the dimer join. A model of the full p53 tetramer–DNA complex suggests that contacts between two dimers are less frequently mutated.

Optics: The mirror vanishes

Phys. Rev. Lett. 97, 053902 (2006)

If you shine light on a silver mirror, you'd expect it to bounce back. But Ian Hooper and his colleagues at the University of Exeter, UK, have shown that, in the right circumstances, light can pass straight through.

They sandwiched a 40-nanometre-thick silver film between two glass prisms coated with thin films of zinc sulphide. In this arrangement, the refractive indices at the material interfaces are well matched, rendering the silver film partially invisible to a light beam — in the same way that a drinking glass may vanish when immersed in water.

Transmission peaked at about 35% for visible wavelengths — not perfect, because the zinc sulphide absorbs some light. The effect could be useful for making metal electrodes transparent in light-emitting diodes.

Neurobiology: After the bomb

Credit: NATL NUCLEAR SECURITY ADMIN./NEVADA SITE OFFICE

Proc. Natl Acad. Sci. USA 103 12564–12568 (2006)

A legacy of nuclear-bomb tests is helping to settle controversy about whether new neurons are born in the brain's neocortex throughout our lives.

The level of carbon-14 in the atmosphere (and thus the human body) was more or less doubled by Cold War bomb tests, then dropped after the 1963 Test Ban Treaty. A team led by Peter Eriksson of Gothenburg University and Jonas Frisén of the Karolinska Institute, Stockholm, Sweden, measured carbon-14 in autopsy tissue of people born before and after the bomb tests. Levels of the isotope in neocortex neurons corresponded to atmospheric levels at the time of the person's birth, suggesting that almost all neurons are born at that time. This supports Frisén's earlier work.

Furthermore, new neurons did not appear to be generated in the neocortex of cancer patients who took a compound that marks dividing cells.

Journal club

Laurence Eaves

University of Nottingham, UK

A semiconductor physicist sees graphene transistors as a triumph for small-scale research.

Reading about a major scientific breakthrough from a small university group always gives me particular pleasure. Funding agencies tend to favour big headline-grabbing initiatives, but many research successes in condensed-matter physics, such as the development of magnetic resonance imaging and polymer light-emitting diodes, have come from small groups funded initially by modest grants.

Recent work from Manchester University, UK, on the electronic properties of graphene further demonstrates what a small, dedicated team can achieve. In 2004, researchers there fabricated a transistor from a few atomic layers of graphite (K. S. Novoselov et al. Science 306, 666–669; 2004), a cheap and widely available material.

Why, I wondered, hadn't somebody thought about doing this before? And why were the electronic properties so good?

With the benefit of hindsight, one can see why it works. The Manchester team had the skill and patience to cleave graphite right down to a single atomic layer. An atomic sheet of graphite (graphene) contains few free electrons, so it is easy to control the current flow. Also, the electrons in graphene have a curious property: they move as though they have almost no inertial mass, so the device can act as a fast switch.

Recently, the same group showed that small ripples in graphene, like rucks in a carpet, scatter electrons via a subtle quantum effect (S. V. Morozov et al. Phys. Rev. Lett. 97, 016801 1–4; 2006). By improving their fabrication technique, the team succeeded in removing the ripples, enhancing further the device characteristics.

Longer-term, we should see new applications of graphene transistors in electronics. Funding agencies and policy-makers, please note!