Plant Molecular Biology 11:791-794 (1988) © Kluwer Academic Publishers, Dordrecht - Printed in the Netherlands 791 The Agrobacterium tumefaciens T-DNA gene 6 b is an onc gene P. J. J. Hooykaas, H. den Dulk-Ras and R. A. Schilperoort Department of Plant Molecular Biology, Leiden University, Wassenaarseweg 64, 2333 AL Leiden, Netherlands Received 29 June 1988; accepted in revised form 12 September 1988 Key words." Agrobacterium tumefaciens, tumor induction, onc gene, T-DNA Abstract In this article it is shown that the T-DNA of Agrobacterium tumefaciens contains besides the well-known cyt and aux genes another gene with an oncogenic effect in plants. The gene in question is called 6 b and causes the formation of small tumors in plant species such as Nicotiana glauca and Kalanchoe tubiflora. Introduction The soil bacterium Agrobacterium tumefaciens is a phytopathogen by virtue of its ability to transform normal plant cells into tumor cells. This eventually results in tumor formation in dicotyledonous plants. The molecular mechanism underlying this bacte- rium-plant interaction has been described in some detail (for a recent review see [11]). Transfer of a piece of oncogenic DNA, the T-DNA, originating from the large, bacterial Ti plasmid, to plant cells at the infection sites is the key event of the process. The T- DNA contains onc genes, the expression of which is responsible for the tumorous character of the trans- formed plant cells. Mutational analysis has shown that three one genes are of prime importance, viz. the genes cyt (ipt), auxl (iaaM) and aux2 (iaaH). These genes code for enzymes involved in the biosynthesis of the cytokinin isopentenyl-AMP (cyt) and the aux- in indole acetic acid (auxl, aux2). Mutations in Agrobacterium T-DNA genes other than cyt, auxl and aux2 did not lead to avirulence [4]. Therefore, it is questionable whether any of the other genes that naturally are present in the T-DNA, are involved in tumorigenesis. Only some indirect data point to a possible accessory or regulatory role of some of these genes (e.g. T-DNA genes 5 and 6 a, 6 b) in tumorige- nicity. Mutations in the area covering genes 6 a and 6 b of the wild-type octopine Ti plasmid were found to lead to an increased size of tumors induced on kalanchoe [4] but similar mutations in the nopaline Ti plasmid had no such effect [9]. Recent data showed that gene 6 a determines a permease system for the excretion of octopine and related opines from transformed plant cells [12], which makes a role of this gene in determining tumor size difficult to un- derstand. In this paper we show that T-DNA gene 6 b is an onc gene by itself, capable of inducing tumor formation in a limited set of plant species. Materials and methods Bacterial strains and plasmids Escherichia coli strain KMBLlI64 thi pro lac (P, van de Putte) was used as a recipient for plasmid con- structs in transformation experiments. Transformed plasmids were subsequently transferred to Agrobac- terium via conjugation in triparental matings with HB101 (pRK2013). The binary vector pBinl9, which contains a Km r marker, was described by Bevan [2].