CHARACTERIZATION OF AN UNUSUAL SENSOR GENE (VIRA) OF AGROBACTERIUM

Previous studies have shown that the virulence(vir) genes of Agrobacte rium tumefaciens strain KU12 are induced by a unique set of phenolic c ompounds that are non-functional in most strains of Agrobacterium. Fur ther, strain KU12 is not induced by phenolic compounds that induce the vir genes in other strains. Previous studies have shown that these di fferences in inducing activity result from differences in the sensor p rotein for these signal molecules, the VirA protein. To gain some unde rstanding of the basis for these differences in sensing ability, we se quenced the entire vir A locus of pTiKU12, including its promoter regi on and compared this sequence with five different published vir A sequ ences that respond in different ways to inducing compounds. The vir A gene of KU12 is composed of an open single reading frame coding for 85 1 aa. At the aa level, the VirA protein of pTiKU12 is 45, 45, 49, 49 a nd 64% identical to the VirA proteins from pTiA6, pTi15955, pRiA4, pTi C58 and pTiAg162, respectively. The transcription start sites of pTiKU 12 and pTiA6 virA genes differ significantly when mapped by primer ext ension. Unlike all other vir genes, except the virA gene of pTiAg162, pTiKU12 virA is constitutively expressed, and its synthesis is not ind uced by phenolic compounds. The lack of induction is accounted for by the fact that the promoter region does not have the conserved VirG-bin ding dodecadeoxynucleotide sequence (vir-box) that was previously iden tified in all promoter regions of inducible vii genes. (C) 1998 Elsevi er Science B.V.