CONSERVED CIS-ACTING PROMOTER ELEMENTS ARE REQUIRED FOR DENSITY-DEPENDENT TRANSCRIPTION OF AGROBACTERIUM-TUMEFACIENS CONJUGAL TRANSFER GENES

Ti plasmids of Agrobacterium tumefaciens, in addition to transferring oncogenic DNA to the nuclei of infected plant cells, can conjugally tr ansfer between agrobacteria. Conjugation of wide-host-range octopine-t ype Ti plasmids requires a tumor-released arginine derivative called o ctopine. Octopine stimulates expression of the traR gene, whose produc t directly activates other tra genes in the presence of an acylated ho moserine lactone called Agrobacterium autoinducer (AAI). We have local ized the transcription starts of three tra promoters and find conserve d elements (tra boxes) at virtually identical positions upstream of ea ch promoter. Disruption of these tra boxes abolished induction of each promoter. Deletion analysis of the traI promoter indicates that tra b oxes are the only upstream elements required for transcriptional activ ation. Since Ti plasmid donor cells both produce and respond to AAI, w e tested whether expression of tra promoters was enhanced by high conc entrations of bacteria. Both tra gene expression and conjugation itsel f were strongly stimulated either by high donor densities or by exogen ous AAI.