Figure 43-4: Plasmid pLT7K and its use. An illustration of the use of pLT7K to clone a gene (R) specifying a restriction endonuclease in the absence of its cognate modification enzyme. At 37°C in the absence of IPTG the Lac repressor binds to the lac operator, and transcription from the T7 promoter is minimal; the lambda temperature-sensitive repressor is inactivated, and transcription from pR antagonizes any residual transcription of gene R from the T7 promoter. At 30ºC, in the presence of IPTG, transcription of gene R occurs from the T7 promoter, and transcription from pR, the opposing promoter, is repressed. This figure is taken with permission from Kong et al. (44).