The transposase of the bacterial insertion sequence IS1 is normally expressed 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 high transposition activity and should express the transposase, InsAB', constitutively without frameshifting. Unexpectedly, a second protein species was observed to be expressed from this mutant. We demonstrate here that this protein, InsA*, results from continued frameshifting on the modified frameshift motif. The protein retains the activities of the repressor InsA. Its elimination, by further modification of the frameshift motif, results in a further increase in various transposition activities of IS1. These results support the hypothesis that a single IS1-encoded protein, InsAB', is necessary for transposition.