CODON REASSIGNMENT IN CANDIDA SPECIES - AN EVOLUTIONARY CONUNDRUM

A number of Candida species translate the standard leucine CUG codon a s serine rather than as leucine. Such codon reassignment in nuclear-en coded mRNAs is unusual and raises a number of important questions abou t the origin of the genetic code and its continuing evolution. In part icular we must establish how a codon can come to be reassigned without extinction of the species and what, if any, selective pressure drives such potentially catastrophic changes. Recent studies on the structur e and identity of the novel CUG-decoding tRNA(Ser) from several differ ent Candida species have begun to shed light on possible evolutionary mechanisms which could have facilitated such changes to the genetic co de. These findings are reviewed here and a possible molecular mechanis m proposed for how the standard leucine CUG codon could have become re assigned as a serine codon.