Phenotypic suppression by incorporation of an alien amino acid.

Azaleucine is a naturally occurring amino acid antibiotic that can be incorporated into proteins by mimicking leucine. In spite of its highly toxic character, the contrast between the hydrophobic side-chain of leucine and the basic side-chain of azaleucine suggested a mechanism for rescuing certain mutants. We constructed a thymidine auxotrophic mutant of Escherichia coli by replacing an arginine residue with leucine in the catalytic centre of thymidylate synthase, and indeed showed that activity could be restored by incorporation of azaleucine. This result extends the current scope of phenotypic suppression to mischarging with amino acid analogues. Microbial strains with a clear-cut requirement for an additional amino acid, as reported here, should be instrumental for widening the genetic code experimentally.