Mutations in the NAM2 genes of Saccharomyces cerevisiae and S. douglasii are clustered non-randomly as a result of constraints on the nucleic acid sequence and not on the protein

Summary We present a statistical study of the nature and distribution of mutations along the NAM2 gene coding for the mitochondrial leucyl tRNA synthetase in Saccharomyces cerevisiae and S. douglasii (Herbert et al. 1988). Two important facts are observed: (1) the relative frequency of transitions and transversions is the same among silent substitutions and replacements. (2) The two kinds of mutations (silent substitutions and replacements) are distributed in the same way along the gene. This distribution is not random; the mutations are clustered and the clusters are regularly spaced along the gene. The NAM2 gene offers an example spaced along the gene. The NAM2 gene offers an example of recent divergence. We show that, in this case, the fixation of mutations is the result of genetic drift and of constraints on the nucleic acid sequence and not on that of the protein.