ИСТИНА

We used the comparative genomics approach that combines identification of candidate regulator-binding sites with cross-genomic comparison of regulons, to reconstruct the biochemical pathways and transcriptional regulons for histidine metabolism. Thus, we propose a plausible evolutionary scenario for regulation of histidine biosynthesis genes. The orthologs of HisR repressor were identified in 397 Firmicutes genomes, representing four classes (Bacilli, Clostridia, Negativicutes, and Tissierellia), with the only exception of bacteria from the Erysipelotrichia class. A small group of closely-related Bacillus species as well as another large group of bacteria have lost HisR repressor, which was replaced by functional analogues including T-box- and attenuator-dependent regulative mechanisms. We chose one representative HisR regulator to experimentally assess its specific DNA-binding properties and to test possible small-molecule effectors. We identified DNA binding sites of HisR regulators and reconstructed their regulons in the majority of Firmicutes and other taxonomic lineages. Also, we have shown the ancestor HisR regulatory system being replaced by different RNA regulatory systems including T-box and multiple types of histidine attenuators in certain taxonomic groups of Firmicutes. Moreover, we have experimentally confirmed the histidine to be the most effective in stimulating in vitro binding of HisR to its DNA fragments.