The vir gene products of Agrobactenium tumefaciens carry out the transfer of T-DNA to the plant genome. Effective transcriptional induction of the vir genes by plant signal molecules is controlled by two vir gene products, VirA and VirG. In this study we have identified and cloned a chromosomal region which is also required for vir gene induction. Transposon insertions within this region reduce induction significantly and strongly attenuate virulence, resulting in a restricted host range for infection. The reduction in vir gene transcription can be partially overcome by high concentrations of the inducer molecule acetosyringone. Expression of virG at low pH and low phosphate concentrations, which is independent of plant signals, is not affected by these mutations. Sequence analysis of the region revealed two divergent open reading frames, which we have designated chvE and ORF1. Several transposon insertions mapped in chvE; this resulted in attenuated virulence. chvE codes for a putative protein which is homologous to two periplasmic receptor proteins involved in chemotaxis and uptake of sugars. Whether ORF1 is required for virulence is uncertain. One transposon insertion resulting in avirulence maps in or near the 5' end of ORF1, and several which do not affect virulence map in its 3' end. ORF1 codes for a putative protein which is homologous to a family of transcriptional activator proteins.Strains of Agrobacterium tumefaciens containing a Ti plasmid can incite crown gall tumors on most dicotyledonous and some monocotyledonous plants. Two regions of the Ti plasmid, the transferred DNA (T-DNA) and virulence (vir) regions, are necessary for tumor formation. During infection of wounded plant tissue, the T-DNA is transferred and integrated into the plant nuclear DNA (6,49), where its expression results in crown gall tumor formation (12,18,53). In addition to the Ti plasmid virulence genes, chromosomal genes chvA, chvB, and exoC are required for complete virulence (3, 11). The functioning of these genes is required for attachment of the bacteria to plant cells.Transcription of the vir genes is induced by phenolic compounds, such as acetosyringone, synthesized by wounded plant cells (43,44). The induction system is controlled by two of the vir genes, virA and virG (45, 54 The host range for Agrobacterium infection is determined largely by the T-DNA and vir regions of the Ti plasmid (28,29,35,50). Increasing the expression of certain vir genes increases the virulence and host range of the bacteria (25). However, several observations have indicated that chromosomal genes are also involved in determining host range (17,20). In this study, we found that chromosomal host range mutants originally isolated by Garfinkel and Nester (17) are defective in vir gene induction.
MATERIALS AND METHODSBacterial strains, plasmids, and media. Escherichia coli DH5(x, HB101 (1), and SF800 (22)
