## 1 Introduction  
The soil bacterium _Corynebacterium glutamicum_, along with _Escherichia coli_, is one of the most commonly used basic microorganisms in creating amino acid producers, including essential branched-chain amino acids (BCAAs)--L-valine, L-leucine, and L-isoelucine. L-valine is used in food and pharmaceutical industries, medicine, and cosmetics, but primarily as a feed additive for animals. Acetohydroy acid synthase (AHAS) is a key enzyme in the first step of the branched-chain amino acid synthesis pathway. In bacteria, the level of the transcription of operons providing amino acid biosynthesis is often controlled by a negative feedback mechanism through so-called attenuators [1]. The functioning of attenuators is associated with the synthesis of the leader peptide by ribosomes and the possibility of the formation of alternative secondary RNA structures in the regulatory region. When there is a lack of certain amino acids in the cell, the ribosome slows down at the codons corresponding to these amino acids, which allows the secondary structure of RNA to form antiterminator hairpins before terminator hairpins are formed. Conversely, the rapid passage of certain amino acid codons by ribosomes when there is an excess of these amino acids in the cell leads to the formation of RNA terminator hairpins that block transcription. In _C. glutamicum_, the _ilvBNC_ operon, which encodes the first two enzymes of the biosynthesis of branched-chain amino acid (BCAAs: isoleucine, leucine, and valine), is subject to such regulation via a negative feedback mechanism [2]. The genes _ilvB_ and _ilvN_ encode subunits of the acetohydrox acid synthase enzyme (AHAS, EC 2.2.1.6), and the _ilvC_ gene encodes acetohydrox acid isomeroreductase (AHAIR, EC 1.1.1.86) [3; 4]. A low concentration of one or all three amino acids in the medium leads to a twofold increase in the activity of the operon [4]. According to current concepts, the _ilvB_, _ilvN_, and _ilvC_ genes in _C. glutamicum_ constitute a combined _ilvBNC_ operon, which contains three active promoters (Figure 1), and their expression leads to the formation of transcripts of varying lengths--_ilvBNC_, _ilvNC_, and _ilvC_[3]. The full-length transcript _ilvBNC_ starts 292 bp upstream of the first structural gene. The transcription initiation site +1 is only one nucleotide away from the start codon of the leader peptide [4]. It was also shown [4] that this 292 bp long region contains a sequence encoding the leader peptide IlvL of 15 amino acids and a region where the formation of secondary structures is possible, due to the presence of direct and reverse repeats. [PERSON] et al. [4] argue for the additional presence of a fourth short transcript _ilvL_ approximately 200 bp in length in the region upstream of the first structural gene of the operon. The presence of a short transcript is a characteristic feature of attenuator-modulated transcriptional control. The leader peptide MTIIRLVVVTRLP contains several codons for BCAAs: two for isoleucine (I), three for valine (V), and two for leucine (L). The replacement of three valine residues in the leader peptide with alanine residues leads to loss of valine-dependent expression, the replacement of the ATG start codon of the leader peptide with AGG reduces the expression of the operon by 80%, and a leader peptide LacZ fusion results in active beta-galactosidase [4]. All of these findings indicate the existence of a translation-dependent attenuator mechanism of transcription regulation of the _ilvBNC_ operon, similar to that for the translation-dependent attenuation operons _trp_ and _his_ in _E. coli_ and _Salmonella_[5]. The described regulatory region is a conserved sequence found in the genome of various species of the genus _Corynebacterium_, which suggests the existence of the same mechanism for regulating the expression of the _ilvBNC_ operon [6]. However, the role of RNA secondary structures in the regulation of _ilvBNC_ is not completely clear.