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Science 325 (5947): 1505-1506

Copyright © 2009 by the American Association for the Advancement of Science

Cell Biology

Expanding Functionality Within the Looking-Glass Universe

Steven R. Blanke

After she stepped through a looking glass into a mirror image of her own world, Lewis Carroll's adventuress Alice soon recognized that the two worlds were perhaps not so similar after all. She intuits that something about the very make-up of "looking-glass milk" was fundamentally different from that of the wholesome beverage she typically enjoyed (1). Indeed, nature has stocked our universe with biological substances that exist as two mirror image forms of one another. Nineteen of the 20 naturally occurring amino acids that make up proteins are "chiral," meaning that each can be arranged in two orientations around a central carbon atom. The result is a mixture of "mirror-image compounds" called L- and D-amino acids, which cannot be superimposed (see the figure), much in the way a person's left and right hands are not superimposable. One of the great mysteries of life has been the emergence of a strictly "left-handed" protein world where great attention has been paid to the L-amino acid building blocks (24), while D-amino acids are largely regarded as red-headed cousins who are easy to ignore. On page 1552 of this issue, Lam et al. challenge this perception, describing a role for D-amino acids in controlling bacterial responses to environmental cues (5).

Department of Microbiology and the Institute for Genomic Biology, University of Illinois, Urbana, IL 61810, USA.

E-mail: sblanke{at}

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