A Brief Overview of LacI-Family Transcriptional Regulators in Bacteria

Various regulatory mechanisms control the initiation of transcription to regulate gene expression. Several families of transcription factors (TFs) regulate this biological process. The lactose repressor protein (LacI) family is the second-largest family of TFs in bacteria and archaea. Members of the LacI family have been reported to regulate gene transcription in Enterobacteria, Proteobacteria, and Firmicutes. This family has more than 1,000 members that have been characterized, as well as many putative homologs. In terms of their characteristic function, the LacI family members are global regulators of sugar metabolism. This family perceives sugar effectors and, in response, controls the expression of the genes involved in the use of carbohydrates. Some members of the LacI family control the expression of specific genes and act as repressors or activators, although the majority of LacI family members are repressors. The presence of an inducer and the location of the operator, whether upstream (O3) or downstream (O1, O2) of the target gene, affect the LacI regulation mechanism. The interaction between the operator and LacI relies on the helix-turn-helix (HTH) motif in the LacI protein. The (T)G-A-CG-T-C(A) sequence in the operator, which contains the conserved core CG group, is recognized by LacI family TFs as the critical motif.