The ilvBNC operon of Corynebacterium glutamicum encodes acetohydroxy acid synthase and isomero-reductase, which are key enzymes of L-isoleucine, L-valine and L-leucine syntheses. In this study we identified the transcript initiation site of ilvBNC operon 292 nucleotides in front of the first structural gene, and detected the formation of a short transcript from the leader region in addition to the full length transcript of the operon. This identifies the control of ilvBNC transcription by an attenuation mechanism involving antitermination. Mutations in the leader region were made and their effect on the operon expression in ilvB'lacZ fusions was quantified, Although a presumed leader-peptide-coding region is only one nucleotide away from the transcript initiation site determined, there is clear evidence to support the formation of this leader peptide: (i) the substitution of initiation codon ATG of the peptide by AGG reduced lacZ expression of the appropriate fusion construct to 19%; (ii) the replacement of three subsequent Val codons by Ala codons resulted in the loss of Val-dependent expression; and (iii) a leader peptide LacZ fusion resulted in active beta -galactosidase. Based on these results, it is concluded that transcription of ilvBNC is controlled by a translational-coupled attenuation mechanism. The absence of a ribosome binding site for leader peptide formation means that additional mechanisms may contribute to the transcription control at the decoding initiation step in the leader peptide formation.