Amino Acid Replacements and the Genetic Code

Abstract

The genetic code is now sufficiently well understood from in vitro studies (Nirenberg et al. and Khorana et al., this volume) to permit a meaningful examination of amino acid replacement data in an effort to identify those factors that restrict the amino acid changes observed as a consequence of single mutational events. The pattern of amino acid substitutions that have been detected both in mutational studies with well-characterized proteins (Wittmann et al., this volume; Yanofsky, 1965; Tsugita and Fraenkel-Conrat, 1962; Funatsu and Fraenkel-Conrat, 1964; Weigert and Garen, 1965) and in naturally occurring variant forms of proteins (see Crick, this volume) suggests that each mutational event leading to an amino acid replacement involves a single nucleotide change, i.e., the codons corresponding to the amino acids that replace one another are identical at two of the three nucleotide positions. If this is the case, only certain amino acids could replace any given...