USE OF T7 RNA-POLYMERASE IN AN OPTIMIZED ESCHERICHIA-COLI COUPLED IN-VITRO TRANSCRIPTION-TRANSLATION SYSTEM - APPLICATION IN REGULATORY STUDIES AND EXPRESSION OF LONG TRANSCRIPTION UNITS

An Escherichia coli coupled in vitro transcription-translation system has been modified to allow efficient expression of genes under the con trol of a T7 promoter. We describe both the characterization and use o f two S30 crude extracts prepared from E. coli, namely S30 BL21(DE3) ( containing endogenous T7 RNA polymerase) and S30 BL21 (supplemented wi th exogenous T7 RNA polymerase). Since transcription by the highly act ive T7 RNA polymerase is known to overload the translational machinery of E. coil, the ratio between mRNA and ribosomes has to be regulated in the coupled in vitro system. For this purpose, the level of mRNA is controlled by varying the amount of DNA template (S30 extract with en dogenous T7 RNA polymerase) or by limited amounts of exogenously added T7 RNA polymerase. The coupled in vitro system described in this pape r provides two especially useful applications. First, it is most suita ble for studying the regulation of gene expression in vitro, second, i t can be used to express DNA templates carrying up to 10 genes. We sho w that genes which are not well expressed in E. coli in vivo because o f unfavourable codon usage or plasmid instability are synthesized effi ciently in the coupled in vitro system.