3
artists have also utilized explosives in their work, but the results have gener- ally been unpredictable. Now Evelyn Rosenberg, a sculptor working at the New Meξco Institute of Mining and Technology, has harnessed the energy of explosives to create predetermined bas-reliefs in brass, copper and stain- less steel. Rosenberg's first attempts were dis- appointing, she wrote recently in Leo- nardo, a journal of technology in the arts. Sometimes the explosives blew holes through the metal or flung the materials into the air. After 80 trials she was finally able to manipulate such variables as the thickness of the metal and the energy of the explosion e power of plastic explosives can be harnessed to sculpt metal bas-reliefs called detonographs LOSS sed and molded stainless steel, brass and copper to form this four-foot-wide bas-relief tit led Forces and Signs. Evelyn Rosenberg created it for Gaitersburg Junior High School in Maland. e photograph is by Jer Goffe. 20 SCIENTIFIC ECAN Februa 1989 to create sculptures, which she calls detonographs. Rosenberg has now produced 70 detonographs, seven of which hang above the entrance to the New Meξco Museum of Natural Histo- ry in Albuquerque. To create a detonograph, Rosenberg first sculpts a bas-relief in plaster to form a mold. She covers the mold with a three-by-four-foot panel of brass, copper or stainless steel. On the metal panel she sometimes places thin col- ored metallic foils and such objects as cloth, string or leaves. At a blast site in Socorro, N.Mex., a techniCian covers the layers with Du Pont's (-1 Deta- sheet, a plastic explosive composed of pentaerythritol tetranitrate, cellulose nitrate and a plastic bipder. en the technician detonates this explosive sandwich (generating a 20- foot fireball), the elosion drives the metal panel into the mold and thereby reproduces the bas-relief in metal in the instant before the mold shatters. The tremendous pressure of the ex- plosion clads the panel with the metal foils, which provide colored accents; the cloth, string or leaves are driven into the panel at 6,800 meters per second and provide texture by leav- ing their imprint before disintegrating in the flames. To complete the detono- graph Rosenberg cleans and polishes the panel. though works similar to detono- graphs are made by die-stamping, casting at a foundry or repoussage (in which soft metal sheets are shaped by hammering), Rosenberg maintains that her method is more suitable for individual works of art, is less time- consuming and is relatively inex- pensive. Her method also makes it possible to clad one kind of metal with another, something that cannot be accomplished employing the other techniques. She comments: "My ex- pression as a sculptor is reinforced by the magic, primitive quality of explo- sive art." -Russell Ruthen BIOLOGIC SCIENCES POU! Goes the Homeobox Developmental DNA sequences are found in puzzling places T he homeobox, once thought to be the holy grail of developmen- tal biology, has recently become a tric k ier cup to drink from. The ho- meobox is a short segment of DNA found in genes that appear to control key events in the embryonic develop- © 1989 SCIENTIFIC AMERICAN, INC

POU! Goes the Homeobox

  • Upload
    john

  • View
    213

  • Download
    0

Embed Size (px)

Citation preview

Page 1: POU! Goes the Homeobox

artists have also utilized explosives in their work, but the results have gener­ally been unpredictable. Now Evelyn Rosenberg, a sculptor working at the New Mexico Institute of Mining and Technology, has harnessed the energy of explosives to create predetermined bas-reliefs in brass, copper and stain­less steel.

Rosenberg's first attempts were dis­appointing, she wrote recently in Leo­nardo, a journal of technology in the arts. Sometimes the explosives blew holes through the metal or flung the materials into the air. After 80 trials she was finally able to manipulate such variables as the thickness of the metal and the energy of the explosion

The power of plastic explosives can be harnessed to sculpt metal bas-reliefs called detonographs

EXPLOSIVES fused and molded stainless steel, brass and copper to form this four-foot-wide bas-relief titled Forces and Signs. Evelyn Rosenberg created it for Gaitersburg Junior High School in Maryland. The photograph is by Jerry Goffe.

20 SCIENTIFIC AMERICAN February 1989

to create sculptures, which she calls detonographs. Rosenberg has now produced 70 detonographs, seven of which hang above the entrance to the New Mexico Museum of Natural Histo­ry in Albuquerque.

To create a detonograph, Rosenberg first sculpts a bas-relief in plaster to form a mold. She covers the mold with a three-by-four-foot panel of brass, copper or stainless steel. On the metal panel she sometimes places thin col­ored metallic foils and such objects as cloth, string or leaves. At a blast site in Socorro, N.Mex., a techniCian covers the layers with Du Pont's (-1 Deta­sheet, a plastic explosive composed of pentaerythritol tetranitrate, cellulose nitrate and a plastic bipder.

When the technician detonates this explosive sandwich (generating a 20-foot fireball), the explosion drives the metal panel into the mold and thereby reproduces the bas-relief in metal in the instant before the mold shatters. The tremendous pressure of the ex­plosion clads the panel with the metal foils, which provide colored accents; the cloth, string or leaves are driven into the panel at 6,800 meters per second and provide texture by leav­ing their imprint before disintegrating in the flames. To complete the detono­graph Rosenberg cleans and polishes the panel.

Although works similar to detono­graphs are made by die-stamping, casting at a foundry or repoussage (in which soft metal sheets are shaped by hammering), Rosenberg maintains that her method is more suitable for individual works of art, is less time­consuming and is relatively inex­pensive. Her method also makes it possible to clad one kind of metal with another, something that cannot be accomplished employing the other techniques. She comments: "My ex­pression as a sculptor is reinforced by the magic, primitive quality of explo­sive art." -Russell Ruthen

BIOLOGICAL SCIENCES

POU! Goes the Homeobox Developmental DNA sequences are found in puzzling places

The homeobox, once thought to be the holy grail of developmen­tal biology, has recently become

a trickier cup to drink from. The ho­meobox is a short segment of DNA found in genes that appear to control key events in the embryonic develop-

© 1989 SCIENTIFIC AMERICAN, INC

Page 2: POU! Goes the Homeobox

The Tandy lOOOTL

T he Tandy 1000 TL is a powerful computer for personal and business use. Its 80286 microprocessor gives you extraordinary speed and process­ing power. Plus, the 1000 TL comes with MS-DOS and the DeskMate Graphical User Interface built in, so you can be up and running in sec­onds, using plain-English commands.

DeskMate features ten applications that let you write reports and letters, prepare budgets, file, draw colorful pictures, create and play back songs and more. Plus, there's PC-Link� an online information service.

You also get the latest in computer­audio technology. When you use DeskMate's sound editor, you can re­cord and edit voice, music or any ana­log source onto diskettes.

T he 640K Tandy 1000 TL comes with a 311z" disk drive and has room for an additional 311z" and 51/4" drive. A parallel printer adapter, RS-232 serial port, two joystick ports, a clock/calendar and five expansion slots are all standard. You also get a lOI-key enhanced keyboard for the ultimate combination of power, ease of use and affordability.

Tandy Computers: Because there is no better va1ueT� MS-OOS/Reg. TM Microsoft Corp. PC-Link/Reg. TM Quantum Computer Services.

286 power with MS--DOS® and DeskMate®

built in.

T he new generation Tandy 1000 TL. From the best-selling family of PC Compatibles made in America.

.. - - -----. • Send me a 1989 RSC·20

• Computer Catalog

• Mail to: Radio Shack, Dept. 89-A-921 • 300 One Tandy Center. Fort Worth, TX 76102

• Name • I

Address

• City __________ _

• State ZIP • a.;h';' ______ .I ladle IhaeK The Technology Store™

A DIVISION OF TANDY CORPORATION

© 1989 SCIENTIFIC AMERICAN, INC

Page 3: POU! Goes the Homeobox

ment of a very wide range of species. It has been assumed that homeobox proteins (the proteins that are encod­ed by such genes) function by bind­ing to DNA and thereby controlling gene expression, but hard evidence was lacking.

Now several groups report conclu­sive evidence that homeobox proteins do bind to DNA and regulate the tran­scription of RNA molecules (the first step in gene expression). Yet the new findings were made in an unexpected context: not in the differentiating cells of an embryo but in the mature tissues of several mammalian species. It ap­pears that some homeobox proteins determine the fate of mature cells rather than embryonic ones. Thus while the mechanism of the homeo­box has been put on a sounder foot­ing, its overall role has been consider­ably complicated.

The homeobox was first identified in 1983 as a common region in sever­al genes that control the division of the embryo of the fruit fly Drosophila into segments and help to determine the segments' fate (which segment be­comes a leg as opposed to becoming a wing, for example). Since that discov­ery many additional homeobox-con­taining genes have been found in Dro­sophila, and similar DNA segments have been implicated in the embryon­ic development of worms, frogs and mice. In all these cases genes contain­ing the homeobox seem to serve as "master" genetic elements, interact­ing with a cascade of other genes to send a cell down a specific pathway of development.

The three mammalian genes that have recently been shown to include the homeobox act in a different con­text. Each of these genes encodes a "transcription factor," which, by bind­ing to a speCific stretch of DNA, influ­ences the rate at which messenger RNA molecules are transcribed. The transcription factors are known as OCT-I, OCT-2 and Pit-I, and each acts in a characteristic range of tissues. At least two of the transcription fac­tors-OCT-2 and Pit-I-serve to deter­mine the final identity of a particular set of mature tissues.

OCT-2 is made in immune-system cells known as B cells; it causes them to produce large quantities of anti­bodies as they develop into the ma­ture type called the plasma cell. The transcription factor has its effects by triggering immunoglobulin genes, which encode the components of anti­bodies. It had been known that OCT-2 acts by binding to DNA In a recent paper in Nature a group led by Robert

G. Roeder of Rockefeller University re­ports that they have cloned the gene for OCT-2 and shown that it contains a homeobox. This result, according to the investigators, is "surprising but significant."

Pit-I, on the other hand, has its ef­fects in the pituitary, where it appar­ently causes two closely related cell populations to make their characteris­tic hormones: growth hormone and prolactin (which sustains lactation). Now two groups at the University of California at San Diego School of Medi­cine (one led by Michael Karin, the other by Michael G. Rosenfeld) report in Cell that they have cloned the Pit-I gene. Both groups agree that the gene for Pit-I includes a homeobox.

The discovery that the homeobox functions in mature cells was not the end of the surprise. Flanking the ho­meobox in each of the three newly characterized genes was another com­mon segment of DNA The homeobox contains 180 nucleotides coding for 60 amino acids; the new region is slightly larger, including enough DNA to encode 75 amino acids. Because it was discovered in the genes for Pit and OCT and also in the roundworm Caenorhabditis elegans in a gene called unc-86 (which has a similar role in specifying the type of mature tis­sues), the newly discovered conserved region has been named POU (pro­nounced "pow").

Although the discovery of the com­bination of POU and the homeobox is intriguing, it clearly raises as many questions as it answers. Among the more significant puzzles are: What is the common thread between the ef­fects of such genes in early devel­opment and in the terminal differen­tiation of mature tissues? What are the evolutionary relations between these two different functions? Of even greater immedi.ilte interest is the question of how the homeobox­POU combination works at the level of molecular detail.

The simple answer to the last ques­tion would seem to be that each ho­meobox protein binds to a unique

DNA sequence, thereby triggering a unique gene or combination of genes. Unfortunately the simple answer ap­pears to be false. Unlike OCT-2 and Pit-I, the OCT-I protein is "ubiqui­tous," or present in not one but many cell types; it triggers the release of proteins synthesized in many differ­ent types of cell. Furthermore, the type-specifying protein OCT-2 and the ubiquitous protein OCT-I bind to the same DNA sequence. A simple model in which the identity of a cell results

22 SCIENTIFIC AMERICAN February 1989

from a unique match between a ho­meobox protein and a particular piece of DNA no longer holds. If the new results are confirmed, then it would seem that the grail holds a beverage of a larger quantity and a lesser potency than once was thought. -John Benditt

A Breed Apart Finicky flies lend credence to a theory of speciation �at is the origin of species?

Conventional wisdom holds that a new species can arise

when members of the same species are separated by geographic barriers; the barriers prevent interbreeding be­tween the two populations, and her­itable differences are established as the populations adapt to their differ­ent environments. Since Darwin's time, however, it has been proposed that species can also originate "sympatri­cally"; that is, some members of a population may become reproductive­ly isolated from the rest of the popula­tion when no physical barriers to mat­ing exist. Now entomologists may have found the first genetic evidence that sympatric speciation does in fact take place.

The evidence comes from studies of a fruit fly called Rhagoletis pomonella. The insect is a parasite of the haw­thorn tree and its fruit, which is com­monly called the thorn apple. The life cycle of the hawthorn fly revolves around the thorn apple: the fruit serves as a rendezvous for courtship and mating, the females lay their eggs in it and the larvae feed on it.

At least some of them do. About 150 years ago R. pomonella began to in­fest apple trees as well as hawthorns. Fly populations diverged into two so­called host races: one that prefers ap­ples and one that prefers hawthorns. A former classmate of Darwin's named Benjamin Walsh first recognized the new apple-fly race and suggested that the differential host preferences could lead to speciation.

In the early 1960's a Harvard Univer­sity graduate student named Guy L . Bush set out t o discredit Walsh's the­ory by demonstrating that apple and hawthorn flies were not becoming two different species. But as Bush learned more about Rhagoletis he began to doubt his own premise. Generally or­ganisms are considered separate spe­cies if they do not interbreed. Bush realized that if the host fruit provides the site for Rhagoletis mating, breed­ing between apple and hawthorn flies

© 1989 SCIENTIFIC AMERICAN, INC