Expression host strains for pETBlue and pTriEx vectors are lDE3 lysogens that also carry the compatible pLacI plasmid, encoding the lac repressor. The pLacI plasmid confers chloramphenicol resistance and provides enough lac repressor to inhibit transcription from the T7lac promoter in the absence of inducer. The NovaBlue host strain is recommended for initial cloning with pETBlue and pTriEx vectors due to its superior transformation efficiency and high yields of high-quality plasmid DNA. Note that these strains are not recommended for use with pET, pACYCDuet, pCDF, pRSF, pCOLADuet or pETcoco vectors. Tuner strains are lacZY deletion mutants of BL21 and enable adjustable levels of protein expression throughout all cells in a culture. The lac permease (lacY) mutation allows uniform entry of IPTG into all cells in the population, which produces a concentration-dependent, homogeneous level of induction. By adjusting the concentration of IPTG, expression can be regulated from very low levels up to the robust, fully induced levels commonly associated with pET vectors. Lower-level expression may enhance the solubility and activity of difficult target proteins. In Tuner(DE3)pLacI, the rare tRNA genes are present on the same plasmid that carries the lac repressor gene. DE3 indicates that the host is a lysogen of λDE3, and therefore carries a chromosomal copy of the T7 RNA polymerase gene under control of the lacUV5 promoter. Such strains are suitable for production of protein from target genes cloned in pET vectors by induction with IPTG.
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