lacIq

lacIq, also written as lacIq, refers to an allele of the lacI gene that produces a higher level of the LacI repressor in Escherichia coli. The q allele typically results from mutations that increase transcription of lacI or enhance its expression, leading to greater intracellular LacI concentrations than in strains with the wild-type lacI gene. This stronger repressor capacity enhances repression of lac-controlled promoters, such as lacUV5, and reduces basal or leaky expression of downstream genes.

In practical use, lacIq is employed to improve control of recombinant gene expression, especially when expressing proteins that are toxic to the host or when a tight on-off switch is required. It is commonly carried on expression plasmids or present in host strains used for recombinant protein production. By increasing repression before induction, lacIq helps minimize unintended expression prior to the addition of inducer (for example, IPTG), thereby improving yield and reducing detrimental effects on cell growth.

lacIq is particularly relevant in systems that rely on lac promoter–driven elements, including T7-based expression setups where lacI represses the lacUV5-driven T7 RNA polymerase. While beneficial in many contexts, the degree of repression provided by lacIq can vary with plasmid copy number, host background, and growth conditions. It is one of several strategies used to achieve tight regulation of gene expression in bacterial systems.