Genetic evidence for IS1 transposition regulated by InsA and the delta InsA-B'-InsB species, which is generated by translation from two alternative internal initiation sites and frameshifting.

Insertion sequence IS1 contains two reading frames, insA and B'-insB, which are responsible for its transposition, and was previously shown to express two proteins. The first, InsA, is the product of insA. The second, InsA-B'-InsB is a fusion of InsA with the product of B'-insB. Synthesis of this protein occurs by a -1 frameshift from the 3' region of the insA frame to the open reading frame B', extending from the 5' end of the insB frame. Here, I have shown genetically that IS1 encodes the third species delta InsA-B'-InsB: delta InsA-B'-InsB uses two alternative initiation codons in the middle of the insA frame, and is produced by a frameshift mechanism similar to that used in InsA-B'-InsB expression. Deletion of the small region preceding these initiation codons resulted in decreased expression of delta InsA-B'-InsB, suggesting that the small region play some role in the translation initiation. Surprisingly, it was found that delta InsA-B'-InsB has a transposase-like function and InsA can stimulate the transposition promoted by delta InsA-B'-InsB, while delta InsA-B'-InsB seemed to bind to the left terminal inverted repeat (IRL) of IS1 and inhibit transposition when it was present in excess, as well as InsA represses transposition. It is likely that IS1 transposition activity depends on the ratio of InsA to delta InsA-B'-InsB. A double missense mutation of the internal initiation codons resulted in decreased cointegration activity, showing that delta InsA-B'-InsB is responsible for transposition but InsA-B'-InsB is probably not. Some IS elements, which also contain two tandem, out-of-phase, overlapping genes, appear to express deleted fusion proteins like delta InsA-B'-InsB, but the functions are unknown. The complex phenomena of transposition and its control found in IS1 may be more general in the other mobile DNAs.