Attenuation in the threonine operon: effects of amino acids present in the presumed leader peptide in addition to threonine and isoleucine.

We have studied in vivo the contribution of amino acids corresponding to codons of the leader sequence other than the so-called "regulatory codons" (for threonine and isoleucine) in the expression of the threonine operon. In the presence of threonine and isoleucine, addition of each amino acid encoded in the proximal part of the leader sequence resulted in a significant decrease of the expression of the operon, over and above that decrease observed in the sole presence of threonine and isoleucine. These effects were cumulative. No such effect was found with the amino acids encoded by the distal part of the leader sequence. These findings are discussed in the light of the current model of attenuation.