Conjugative transposition of Tn916: the transposon int gene is required only in the donor

Conjugative transposition of transposon Tn916 has been shown to proceed by excision of the transposon in the donor strain and insertion of this element in the recipient. This process requires the product of the transposon int gene. We report here the surprising finding that the int gene is required only in the donor during conjugative transposition. We find that Tn916 int-1, whose int gene has been inactivated by an insertion mutation, transposes when a complementing wild-type int gene is present only in the donor during mating. When the int+ gene is present in a plasmid and is expressed from the spac promoter, conjugative transposition is very inefficient. However, when the Int+ function is supplied from a coresident distantly linked Tn916 tra-641 mutant, which is defective in a function required for conjugation, efficient conjugative transposition of Tn916 int-1 occurs. This suggests either that Int is not required for integration of Tn916 in gram-positive bacteria or that the protein is transferred from the donor to the transconjugant during the mating event. When the nonconjugative plasmid pAT145 was present in the donor, it was rarely cotransferred with Tn916. This suggests that complete fusion of mating cells is not common during conjugative transposition.