Inhibition of Agrobacterium tumefaciens oncogenicity by the osa gene of pSa.

The IncW plasmid pSa originally derived from Shigella flexneri completely inhibits the tumor-inducing ability of Agrobacterium tumefaciens when it is resident in this organism. Oncogenic inhibition is mediated through the expression of the osa gene on pSa. This gene is part of a 3.1-kb DNA segment of pSa that contains four open reading frames revealed by sequencing. Specific deletions and TnCAT insertions within this segment localized the oncogenic inhibitory activity to the last open reading frame, orf-4, designated osa (for oncogenic suppression activity). No promoter exists immediately upstream of the coding sequence of osa since TnCAT insertions or deletions into orf-3 caused the loss of oncogenic inhibition. Deletion analysis showed that the promoter of orf-1 is required for osa transcription. The first three orfs have no role in oncogenic inhibition, since osa alone placed under the control of a constitutive Pkm promoter completely inhibited A. tumefaciens oncogenicity. This inhibition of oncogenicity by osa is not limited to a specific host plant but appears to show broad host specificity. Because the osa-encoded product has close homologies to the fiwA-encoded product of the IncP plasmid RP1, osa may be involved in fertility inhibition that would prevent or reduce the formation of stable mating pairs and T-DNA transfer between A. tumefaciens and plants.