IS THE IS1 TRANSPOSASE, INSAB', THE ONLY IS1-ENCODED PROTEIN REQUIREDFOR EFFICIENT TRANSPOSITION

The transposase of the bacterial insertion sequence IS1 is normally ex pressed by inefficient translational frameshifting between an upstream reading frame which itself specifies a transposition inhibitor, InsA, and a second consecutive reading frame located immediately downstream . A fused-frame mutant which carries an additional base pair inserted at the point of frameshifting was constructed. This mutant exhibits hi gh transposition activity and should express the transposase, InsAB', constitutively without frameshifting. Unexpectedly, a second protein s pecies was observed to be expressed from this mutant. We demonstrate h ere that this protein, InsA, results from continued frameshifting on the modified frameshift motif. The protein retains the activities of t he repressor InsA Its elimination, by further modification of the fram eshift motif, results in a further increase in various transposition a ctivities of IS1. These results support the hypothesis that a single I S1-encoded protein, InsAB', is necessary for transposition.