To investigate the mechanism of Tn916-dependent mobilization of plasmids, donors were grown in the presence and absence of tetracycline in the mating medium. The pregrowth of donor in medium containing tetracycline increased conjugative transposition of Tn916 and the transposon-dependent mobilization of pC194 19- to 119-fold in matings between Bacillus subtilis and Bacillus thuringiensis subsp. israelensis. Tn916 and pC194 transferred independently under these conditions. During inter-species matings using isogenic strains, the transfer of Tn916 and plasmid pE194 was detected at a frequency of 1.4x10[superscript]-5 and3.2x10[superscript]-7 transconjugants/donor. Similar frequencies were observed for Tn925-dependent pE194 mobilization. The process of plasmid mobilization by Tn916 and Tn925 is rec-independent. Neither transposon was able to transfer the plasmid pC194 from the recipient (B. thuringiensis) to the donor containing a single chromosomal insertion of the conjugative transposon (B. subtilis). Both plasmids pUB110 and PUB110[delta] m (a mob[superscript]-derivative of pUB110) were mobilized by Tn916. The data suggest that conjugal and mobilizing functions are enhanced by the presence of tetracycline. The mobilization of pC194 does not occur by the complete mixing of the cytoplasm between the donor and the recipient cells. The data also suggest that mobilization of plasmids by Tn916 does not require a functional mob region. Tn916 and Tn925 are functionally similar conjugative transposon.