MOBILIZATION OF SMALL PLASMIDS IN BACILLUS-THURINGIENSIS SUBSP ISRAELENSIS IS ACCOMPANIED BY SPECIFIC AGGREGATION

Mobilizations of pBC16 and pAND006, containing the replicon of the Bac illus thuringiensis subsp. israelensis plasmid pTX14-3, between strain s of B. thuringiensis subsp. israelensis were examined. Transconjugant s appeared after a few minutes and reached a maximum frequency after a pproximately 2 h. Plasmid pBC16 was mobilized at a frequency approxima tely 200 times that of pAND006. However, pAND006 was consistently tran sferred, suggesting that the replicon of pTX14-3 is sufficient to sust ain mobilization in B. thuringiensis subsp. israelensis. A specific pr otease-sensitive coaggregation between strains of B. thuringiensis sub sp. israelensis was found to be unambiguously correlated with plasmid transfer. Two aggregation phenotypes, Agr+ and Agr-, were identified i n this subspecies. Aggregation disappeared when the optical density of the mating mixture at 600 nm exceeded approximately 1, and it did not reappear upon dilution. Aggregation was shown to involve interactions of cells with opposite aggregation phenotypes, and evidence of a prot einaceous molecule on the surface of the Agr- that is cells involved i n aggregation formation is presented. Matings and selection for the pr esence of two antibiotic resistance plasmids followed by identificatio n of the host cell revealed that mobilization was unidirectional, from the Agr+ cell to the Agr-cell. The aggregation phenotype was found to be transferred with high frequency (almost-equal-to 100%) in broth ma tings, and the appearance of Agr - isolates from Agr+ strains suggeste d that the loci involved in aggregation formation are located on a pla smid. No excreted aggregation-inducing signals were detected in the su pernatant or culture filtrate of either the donor, the recipient, or t he mating mixture.