MBE Advance Access published online on May 30, 2003
Molecular Biology and Evolution, doi:10.1093/molbev/msg143
Molecular Biology and Evolution © Society for Molecular Biology and Evolution 2003; all rights reserved
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1 Department of Ecology and Evolutionary Biology, University of Michigan, MI 48109, USA
* To whom correspondence should be addressed. E-mail: jianzhi{at}umich.edu.
A morphological or physiological trait may appear multiple times in evolution. At the molecular level, similar protein functions may emerge independently in different lineages. Whether these parallel functional changes are due to parallel amino acid substitutions has been a subject of debate. Here I address this question using digestive ribonucleases (RNases) of two groups of foregut-fermenting mammals: ruminant artiodactyls and colobine monkeys. The RNase1 gene was duplicated twice in ancestral ruminants at least 40 million years (MY) ago, and it was also duplicated in the douc langur, an Asian colobine, Key Words:
RNase, ruminant, colobine, adaptive evolution, digestion
© 2003 Society for Molecular Biology and Evolution
Original Articles
Parallel Functional Changes in the Digestive RNases of Ruminants and Colobines by Divergent Amino Acid Substitutions
Abstract 
4 MY ago. Following duplication, similar functional changes occurred in the ruminant and monkey enzymes. Interestingly, five amino acid substitutions in ruminant RNases that are known to affect its catalytic activity against double-stranded (ds) RNA did not occur in the monkey enzyme. Rather, a similar functional change in the monkey was caused by a different set of nine substitutions. Site-directed mutagenesis was used to make three of the five ruminant-specific substitutions in the monkey enzyme. Functional assays of these mutants showed that one of the three substitutions has a similar effect in monkeys, another has a stronger effect, while the third has an opposite effect. These results suggest that (1) an evolutionary problem can have multiple solutions, (2) the same amino acid substitution may have opposite functional effects in homologous proteins, (3) the stochastic processes of mutation and drift play an important role even at functionally important sites, and (4) protein sequences may diverge even when their functions converge.
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