IN-VIVO SYSTEM FOR THE DETECTION OF LOW-LEVEL ACTIVITY BARNASE MUTANTS

We report the design of a new tightly controlled barnase system which allows the existence of the barnase gene in host cells without a signa l sequence, When expression of barnase is turned on by gene inversion in vivo, the lethal effect of barnase (or its mutants) is not compromi sed either by coexpression of its polypeptide inhibitor (barstar), or by extracellular secretion, This serves as a rapid, sensitive in vivo test for the detection of any very low residual activity of the barnas e mutants, Active-site mutants His102Lys, Glu73Asp and Arg87Lys, and a mutant which greatly reduces the stability and yield of protein, Arg8 3Lys, produce enough activity to be detectable by this test, In contra st, when expressed on a secretion vector, these mutants do not yield d etectable activity in a solution assay, Truly inactive mutants, such a s those of His102 to Gly, Ala or Leu, were completely harmless when ex pressed in this system.