Mutations that convert the four leucine codons of the Salmonella typhimurium leu leader to four threonine codons

In Salmonella typhimurium, expression of the leucine operon is regulated by a transcription attenuation mechanism. According to a current model of attenuation, elevated expression of this operon requires that a ribosome stall at one of four adjacent codons for leucine on a leader RNA. We used oligonucleotide-directed mutagenesis to convert the four leucine codons of the S. typhimurium leu leader to four threonine codons. Analysis of the resulting mutant operon showed that almost all regulation by leucine had been abolished. The mutant operon was, instead, partially derepressed by a limitation for charged threonine tRNA. These results provide direct evidence for the function for the four leucine codons postulated by the attenuator model. An unexpected observation made during these studies was that the wild-type leu operon was partially derepressed by starvation for threonine.