But because the three-base codons often show up at random, the data were inconclusive. Now evolutionary biologist Laura Landweber and graduate student Rob Knight of Princeton University have done a more careful analysis, looking specifically at where the amino acid arginine binds to random RNA strands generated in several researchers' experiments. If there is no real affinity, they reasoned, codons for arginine will appear as often in the regions where the amino acid does not bind as in regions that arginine homes in on. That suggests, says Landweber, that it's no accident that these codons specify arginine. The arginine evidence is intriguing, says evolutionary biologist Leslie Orgel of the Salk Institute in La Jolla, California.
Researchers are delighted, however, that experimenters are now tackling the question.
Once the code was born, a different kind of pressure, the need to minimize errors, might have refined it. For example, in 1991, evolutionary biologists Laurence Hurst of the University of Bath in England and David Haig of Harvard University showed that of all the possible codes made from the four bases and the 20 amino acids, the natural code is among the best at minimizing the effect of mutations. They found that single-base changes in a codon are likely to substitute a chemically similar amino acid and therefore make only minimal changes to the final protein. Now Hurst's graduate student Stephen Freeland at Cambridge University in England has taken the analysis a step farther by taking into account the kinds of mistakes that are most likely to occur. First, the bases fall into two size classes, and mutations that swap bases of similar size are more common than mutations that switch base sizes.