OCTOPINE-TYPE TI PLASMIDS CODE FOR A MANNOPINE-INDUCIBLE DOMINANT-NEGATIVE ALLELE OF TRAR, THE QUORUM-SENSING ACTIVATOR THAT REGULATES TI PLASMID CONJUGAL TRANSFER

Conjugal transfer of Agrobacterium tumefaciens Tl plasmids is regulate d by two hierarchal signalling systems. Transfer is dependent on a sub set of opines produced by the plant tumours induced by the bacterium. Induction also requires an acyl-homoserine lactone signal, called AAI, that is produced by the bacteria themselves. AAI is the co-inducer fo r TraR, the transcriptional activator required for expression of the t ra regulon. Octopine induces conjugation of the octopine-mannityl opin e-type Ti plasmids by regulating the expression of traR via OccR, the octopine-dependent activator of the opine regulon. We have discovered a second traR-like gene, trlR, on the octopine-mannityl opine-type Ti plasmids pTi15955 and pTiR10. This gene is located in an operon coding for a mannopine transport system and is expressed as part of the mann ityl opine regulon. Sequence analysis indicated that trlR is a framesh ift allele of traR, and the resulting protein lacks the carboxy-termin al domain thought to constitute the DNA-binding region of TraR. Expres sion of trlR inhibited octopine-induced conjugation of pTi15955 and pT iR10 by suppressing the TraR-mediated transcription of the tra and trb operons. Although TrlR had no effect on the expression of traR, TraR activated the expression of trlR. Southern hybridizations indicated th at several other Ti and opine-catabolic plasmids contain more than one copy of genes homologous to traR. We propose that trlR is a dominant negative allele of traR and that TrlR inhibits conjugation by forming inactive heteromultimers with TraR.