Rate of base substitution in mammalian nuclear DNA is dependent on local sequence context

We have analysed the pattern of base substitutions in 40 mammalian processed pseudogenes by comparing them with their cognate functional genes. We find that the rate of base substitution is dependent both on the nature of the mutating base and upon the bases which flank it. Among bases which are not members of CG dinucleotides, bases which are the 5' members of purine-pyrimidine or pyrimidine-purine dinucleotides mutate faster than 5' members of other dinucleotides. These data allow the mutational distance between genes to be estimated more accurately, and suggest some general properties of the mechanism of error prevention in mammalian nuclear DNA.