Controlled-expression cloning vectors

Since the first 370-fold purification of RNase Tl (1), a number of important improvements have been made to increase the yield, purity, and efficacy of purification (31). The improvements in separation techniques include various affinity chromatographies that rely on such adsorbents as 2, 3 -cGMP, aminophenylphos-phoryl-GMP (50), 5 -GMP (40), and 2, 5 -GpG (51). In addition, the RNase Tl gene cloned into an expression—secretion vector efficiently produces the enzyme from . coli hosts (yield of 20 mg enzyme per liter of culture) (49). In this system, the RNase Tl gene is expressed under the inducible control of the lac promoter-operator as a fusion protein with the signal peptide of OmpA, the major outer membrane protein of . coli. The in vivo cleavage of the fusion... 

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