## 1 Introduction
It is also possible that there are additional mechanisms in the regulation of this operon, for example, a ppGpp-sensitive riboswitch mechanism [7] or an additional unknown mechanism of the transcriptional activation of _ilvBN_ expression through 2-ketobutyrate [3; 5]. The starting point of our study was the discovery of a mutation in the regulatory region of the _ilvBNC_ operon of the _C. glutamicum_ strain VF as a result of the full-genome sequencing of some _C. glutamicum_ strains stored in the Russian Collection of Industrial Microorganisms (VKPM) of the National Research Center \"Kurchatov Institute\" (Moscow). These strains were obtained by mutagenesis and selection for their ability to produce valine. This mutation leads to the replacement of guanine with adenine at position 1337948 on the chromosome of _C. glutamicum_ (GenBank: BA000036.3) [8]. The location of this mutation in the secondary structure in the regulatory region of the _ilvBNC_ operon, the absence of other mutations in the structural genes of valine synthesis (_ilvBNC_, _ilvN_, _ilvC_, _ilvD_), and its significant effect on valine production suggest its participation in the realization of the attenuation mechanism of this operon. In this regard, it is of interest to study the structure and mechanism of operation of the attenuator of the _ilvBNC_ operonin _C. glutamicum_ by analyzing mutations affecting the formation of mRNA secondary structures in the attenuator region, the expression of the _ilvBNC_ operon, the activity of enzymes encoded by _ilvBN_ and _ilvC_ genes, and valine production. In the present work, we performed a mutational analysis of the mRNA secondary structures in the regulatory region of the _ilvBNC_ operon identified previously [4; 6], focusing on a region of overlapping alternative hairpin structures. The analysis showed that these hairpin structures mainly function as an antiantiterminator, antiterminator, and terminator. We proved the existence of a transcription terminator in the _ilvBNC_ operon of _C. glutamicum_ and experimentally confirmed its effects on the regulation of _ilvBNC_ operon expression, AHAS enzyme activity, and valine production. It was also shown that the specific location of the antiterminator and terminator hairpins in the _ilvBNC_ operon of _C. glutamicum_ contributes to the effect of rapid switching in the attenuator, which may be important for the coordination of translation and transcription. The reaction catalyzed by AHAS is a key point in the biosynthesis of valine and other BCAAs. The increased expression of _ilvBN_ genes leads to a proportional increase in enzyme activity and, as a consequence, to an increase in valine production, and therefore is of great importance in terms of creating producer strains. Currently, the main methods for increasing _ilvBN_ expression, which are used in constructing such strains, are replacing the intrinsic regulatory region of these genes with a strong constitutive promoter and introducing additional copies of the genes into cells as part of expression plasmids [2]. A deeper understanding of the intrinsic mechanisms of the regulation of the expression of these genes will allow us to find new approaches to modifying the _C. glutamicum_ genome to increase BCAA production. Figure 1: Organization of the _ilvBNC_ operon and its regulatory region in _C. glutamicum_ ATCC 13032. The location of the sequences of the leader peptide IIvL (marked in orange), the structural genes _ilvBN_, _ilvN_, and _ilvC_, as well as the structure of the leader peptide (1337840-1337887 bp), and the 292 bp long regulatory sequence upstream of the _ilvB_ gene (1337839-1338131 bp) according to sequence BA000036.3 are shown. The leader peptide sequence is marked in orange, and the regulatory codons are highlighted—soleucine in blue, valine in crimson, and leucine in purple—and are marked in bold. Designations: P\({}_{1}\), P\({}_{2}\), P\({}_{3}\)—promoters; H\({}_{1}\), H\({}_{2}\), H\({}_{3}\)—inverted repeats indicated by intersecting arrows.