Evidence for a high frequency of simultaneous double-nucleotide substitutions

Point mutations are generally assumed to involve changes of single nucleoti des. Nevertheless, the nature and known mechanisms of mutation do not exclu de the possibility that several adjacent nucleotides may change simultaneou sly in a single mutational event. Two independent approaches are used here to estimate the frequency of simultaneous double-nucleotide substitutions. The first examines switches between TCN and ACY (where N is any nucleotide and Y is a pyrimidine) codons encoding absolutely conserved serine residues in a number of proteins from diverse organisms. The second reveals double- nucleotide substitutions in primate noncoding sequences. These two compleme ntary approaches provide similar high estimates for the rate of doublet sub stitutions, on the order of 0.1 per site per billion years.