Amino Acid Replacements and the Genetic Code

Abstract

The genetic code is now sufficiently well under-stood 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 mu-tational event leading to an amino acid replacement