JACK-OF-ALL-TRADES APPROACH TO SEQUENCING INSTRUMENTS              
   
   Jack Davidson of the Instrumentation and Controls (I&C) Division is
   hedging his bets on sequencing techniques by working on instrumentation
   for three different approaches: conventional electrophoresis film
   methods, a relatively new method known as sequencing by hybridization,
   and his idea for a high-speed gelless system.                    
   
   For the conventional film method, his goal is to develop an inexpensive
   semiautomatic scanner that will read the sequence of DNA bases quickly
   and directly from film without using sophisticated software and
   computing. A prototype now under construction is aimed at individuals
   and smaller groups who don't need or can't justify the more elaborate
   automated systems.        
   
   A couple of unforeseen by-products have resulted from Davidson's
   research in film sequencing methods. Working with Wil Allen of I&C and
   Reggie Miles of the Plant and Equipment Division, he has developed the
   Unsmiler and the Undimmer. The Unsmiler was developed in response to a
   condition known as "smiling" that occurs when the parallel bands on the
   film become tilted, making it difficult to visually determine their
   sequence. The Unsmiler is a hand-operated combination of lenses that
   both magnifies and "unsmiles," or straightens out, the bands, making
   them easier to sequence and improving the definition of closely spaced
   bands.       
   
   The Undimmer tackles the problem of low-contrast, hard-to-read films by
   supplying a method of doubling the contrast of faint images, allowing
   these objects to be detected faster and more easily. The Undimmer uses
   a reflector under the film, rather than a light box. As a result, the
   viewing light comes from above and passes through the film twice rather
   than once, doubling the apparent density of objects on the film. The
   color of the reflector (yellow) is chosen to match the peak of the eye's
   photopic sensitivity, further enhancing the effect. Davidson also
   envisions possible applications of the Undimmer in medical and
   industrial radiography where it is desirable to see faint objects on a
   nearly transparent film background. In a recent demonstration of the
   device for the Oak Ridge Radiology Group, the Undimmer was used to turn
   a discarded, substandard mammogram into a "good, readable film,"
   according to one radiologist.          
   
   In sequencing by hybridization, Davidson is lending his support to the
   research efforts of Bob Foote of the Biology Division. Foote's research
   involves synthesizing all possible combinations of 8-base-long sequences
   of DNA (8-mers) and arranging them on a slide in a known pattern. An
   unknown DNA sequence is then added to the slide where it hybridizes, or
   "sticks," to the corresponding 8-base-long sequences that are already
   there.       
   
   Because the unknown DNA is labeled with a radioactive or fluorescent
   tag, all of the 8-mers that make up the unknown segment should be
   detectable, and, in theory, their order in the unknown DNA could then be
   computed. From this information, the sequence of the entire unknown DNA
   segment can be determined. Davidson, using his "lensless" radiation
   microscope, which employs a combination of scintillators, fiber optics,
   and low-light-level television, is trying to develop a high-resolution,
   position-sensitive detector to locate and identify the hybridized
   8-mers.      
   
   Finally, Davidson, with the help of Diedre Falter of the l&C Division
   and David Donohue, formerly of the Analytical Chemistry Division, is
   working on a high-speed, gel-less sequencing system that could
   potentially improve sequencing speed by orders of magnitude. Instead of
   the slow conventional method of separating DNA fragments in a thin layer
   of gel, the fragments would be separated by a time-of-flight mass
   spectrometer in milliseconds instead of hours. The sequence could then
   be determined by order of finish in a race over a known distance.
   Although much work remains to be done, the approach shows considerable
   promise, as evidenced by related approaches begun independently in other
   laboratories.
   

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   Date Posted:  1/10/94  (ktb)